Brainfuck Interpreter Brackets

For some reason, the bracket code I implemented into a Brainfuck interpreter I'm working on isn't working correctly. Here's my code so far:

'Brainf**k Interpreter v1.0
'By HTV04

OPTION STRICT
ACLS

VAR VER$="1.0 (SB3)"

'--Library--
DEF REPLACE$(S$,O$,N$) 'From YARN.LIB v1.2
 VAR W$,I%,S%
 VAR L%=LEN(O$)
 VAR N%=LEN(N$)
 W$=S$+""
 
 I%=INSTR(W$,O$)
 WHILE I%!=-1
  W$=SUBST$(W$,I%,L%,N$)
  S%=I%+N%
  I%=INSTR(S%,W$,O$)
 WEND
 
 RETURN W$
END

DEF COUNT(S$,U$) 'From YARN.LIB v1.2
 VAR O%,I%,S%
 
 REPEAT
  I%=INSTR(S%,S$,U$)
  INC O%,I%!=-1
  INC S%,LEN(U$)
 UNTIL I%==-1||S%>=LEN(S$)
 RETURN O%
END

DEF BFERROR S$,I
 BEEP 0
 PRINT ""
 PRINT "Syntax error: "+S$
 PRINT "Break at "+STR$(I+1)
END

'----

'--Loading Files--
IF PRGNAME$(3)!="-BFIOPTS.PRG" THEN
 LOAD "PRG3:-BFIOPTS.PRG",0
ENDIF
EXEC 3

'--

'----

'--Constant Setup--
VAR OPTIMIZE=VAR("3:OPTIMIZE")
VAR MEMSIZE=VAR("3:MEMSIZE")
VAR BITWIDTH=VAR("3:BITWIDTH")
VAR SIGNEDINT=VAR("3:SIGNEDINT")

VAR SBOPTIMIZE=VAR("3:SBOPTIMIZE")
VAR SBMEMOF=VAR("3:SBMEMOF")
VAR SBCHR13=VAR("3:SBCHR13")
VAR SBINPUT=VAR("3:SBINPUT")
VAR SBINPUT$=VAR("3:SBINPUT$")
VAR SBINPUTEOF=VAR("3:SBINPUTEOF")
VAR SBWAIT=VAR("3:SBWAIT")
VAR SBWAITCNT=VAR("3:SBWAITCNT")

'----

'--Variable Setup--
VAR I=0
VAR I1=0
VAR I2=0

VAR A

VAR FAILSAFE=0

VAR G$=""
VAR BFCMDS$="+-><[].,"
VAR S$=""

VAR BITMAX=0
VAR BITMIN=0
VAR P=0
VAR BC=0
VAR IN=0

'----

'--Array Setup--
DIM MEM[MEMSIZE]
DIM BRACKET[0]

'--Setup--
PRINT "Brainf**k Interpreter v"+VER$
PRINT "By HTV04"
PRINT ""

IF OPTIMIZE!=0 THEN
 IF OPTIMIZE!=1 THEN
  FAILSAFE=1
 ENDIF
ENDIF
IF MEMSIZE<0 THEN
 FAILSAFE=1
ELSEIF MEMSIZE!=FLOOR(MEMSIZE) THEN
 FAILSAFE=1
ENDIF
IF BITWIDTH<0 THEN
 FAILSAFE=1
ELSEIF BITWIDTH!=FLOOR(BITWIDTH) THEN
 FAILSAFE=1
ENDIF
IF SIGNEDINT!=0 THEN
 IF SIGNEDINT!=1 THEN
  FAILSAFE=1
 ENDIF
ENDIF

IF SBOPTIMIZE!=0 THEN
 IF SBOPTIMIZE!=1 THEN
  FAILSAFE=1
 ENDIF
ENDIF
IF SBMEMOF!=0 THEN
 IF SBMEMOF!=1 THEN
  FAILSAFE=1
 ENDIF
ENDIF
IF SBCHR13!=0 THEN
 IF SBCHR13!=1 THEN
  FAILSAFE=1
 ENDIF
ENDIF
IF SBINPUT!=0 THEN
 IF SBINPUT!=1 THEN
  FAILSAFE=1
 ENDIF
ENDIF
IF SBINPUTEOF!=0 THEN
 IF SBINPUTEOF!=1 THEN
  IF SBINPUTEOF!=2 THEN
   FAILSAFE=1
  ENDIF
 ENDIF
ENDIF
IF SBWAIT!=0 THEN
 IF SBWAIT!=1 THEN
  FAILSAFE=1
 ENDIF
ENDIF

IF FAILSAFE==1 THEN
 PRINT "ERROR: One or more options are set incorrectly:"
 IF OPTIMIZE!=0 THEN
  IF OPTIMIZE!=1 THEN
   PRINT "- OPTIMIZE"
  ENDIF
 ENDIF
 IF MEMSIZE<0 THEN
  PRINT "- MEMSIZE"
 ELSEIF MEMSIZE!=FLOOR(MEMSIZE) THEN
  PRINT "- MEMSIZE"
 ENDIF
 IF BITWIDTH!=0 THEN
  IF BITWIDTH!=1 THEN
   IF BITWIDTH!=2 THEN
    PRINT "- BITWIDTH"
   ENDIF
  ENDIF
 ENDIF
 IF SIGNEDINT!=0 THEN
  IF SIGNEDINT!=1 THEN
   PRINT "- SIGNEDINT"
  ENDIF
 ENDIF
 
 PRINT ""
 
 IF SBOPTIMIZE!=0 THEN
  IF SBOPTIMIZE!=1 THEN
   PRINT "- SBOPTIMIZE"
  ENDIF
 ENDIF
 IF SBMEMOF!=0 THEN
  IF SBMEMOF!=1 THEN
   PRINT "- SBMEMOF"
  ENDIF
 ENDIF
 IF SBCHR13!=0 THEN
  IF SBCHR13!=1 THEN
   PRINT "- SBCHR13"
  ENDIF
 ENDIF
 IF SBINPUT!=0 THEN
  IF SBINPUT!=1 THEN
   PRINT "- SBINPUT"
  ENDIF
 ENDIF
 IF SBINPUTEOF!=0 THEN
  IF SBINPUTEOF!=1 THEN
   IF SBINPUTEOF!=2 THEN
    PRINT "- SBINPUTEOF"
   ENDIF
  ENDIF
 ENDIF
 IF SBWAIT!=0 THEN
  IF SBWAIT!=1 THEN
   PRINT "- SBWAIT"
  ENDIF
 ENDIF
 PRINT ""
 PRINT "Please set the values for these options correctly."
 END
ENDIF

IF SIGNEDINT==0 THEN
 BITMAX=POW(2,BITWIDTH)-1
 BITMIN=0
ENDIF
IF SIGNEDINT==1 THEN
 BITMAX=POW(2,BITWIDTH-1)-1
 BITMIN=POW(2,BITWIDTH)*(-1)
ENDIF

'--Code Minification--
PRINT "Reading... ";
FOR I=1 TO PRGSIZE(1)
 PRGEDIT 1,I
 G$=PRGGET$()
 FOR I1=0 TO LEN(G$)-1
  FOR I2=0 TO LEN(BFCMDS$)-1
   IF G$[I1]==BFCMDS$[I2] THEN
    S$=S$+BFCMDS$[I2]
   ENDIF
  NEXT
 NEXT
NEXT
PRINT "Done."

'----

'--Code Optimization--
IF OPTIMIZE==1 THEN
 PRINT "Optimizing... ";
 REPEAT
  S$=REPLACE$(S$,"+-","")
 UNTIL COUNT(S$,"+-")==0
 REPEAT
  S$=REPLACE$(S$,"-+","")
 UNTIL COUNT(S$,"-+")==0
 PRINT "Done."
ENDIF

'----

'--Interpreter Optimization--
IF SBOPTIMIZE==1 THEN
 S$=REPLACE$(S$,"[-]","A")
 S$=REPLACE$(S$,"[+]","A")
ENDIF

'----

'--Interpreter--
CLS
PRINT "";

FOR I=0 TO LEN(S$)-1
 IF S$[I]=="+" THEN
  INC MEM[P]
  IF MEM[P]>BITMAX THEN
   MEM[P]=BITMIN
  ENDIF
 ENDIF
 IF S$[I]=="-" THEN
  DEC MEM[P]
  IF MEM[P]<BITMIN THEN
   MEM[P]=BITMAX
  ENDIF
 ENDIF
 IF S$[I]==">" THEN
  INC P
  IF P==MEMSIZE THEN
   P=0
  ENDIF
 ENDIF
 IF S$[I]=="<" THEN
  DEC P
  IF P<0 THEN
   P=MEMSIZE-1
  ENDIF
 ENDIF
 IF S$[I]=="[" THEN
  IF MEM[P]==0 THEN
   FOR I=I+1 TO LEN(S$)-1
    IF S$[I]=="[" THEN
     INC BC
    ENDIF
    IF S$[I]=="]" THEN
     IF BC>0 THEN
      DEC BC
     ELSE
      BREAK
     ENDIF
    ENDIF
   NEXT
   IF I==LEN(S$)-1 THEN
    BFERROR "missing ]",I
    END
   ENDIF
  ELSE
   PUSH BRACKET,I
  ENDIF
 ENDIF
 IF S$[I]=="]" THEN
  IF LEN(BRACKET)>0 THEN
   IF MEM[P]!=0 THEN
    I=BRACKET[LEN(BRACKET)-1]
   ELSE
    A=POP(BRACKET)
   ENDIF
  ELSE
   BFERROR "missing [",I
   END
  ENDIF
 ENDIF
 IF S$[I]=="." THEN
  IF MEM[P]!=13 THEN
   PRINT CHR$(MEM[P]);
  ENDIF
 ENDIF
 
 IF S$[I]=="A" THEN
  MEM[P]=0
 ENDIF
NEXT

Any idea on what I'm doing wrong?

 IF S$[I]=="[" THEN
  IF MEM[P]==0 THEN
   FOR I=I+1 TO LEN(S$)-1

I think you are overwriting the value of I here.

That's what I'm trying to do. I is the program pointer, and it has to move around when it detects a bracket.