STICK can yield a velocity of a cursor, not an absolute coordinate.
Try modifying your code as follows.
ACLS X=200:Y=120 SPSET 1,2 WHILE 1 VSYNC 1 STICK OUT SX,SY LOCATE 1,2:?FORMAT$("%03D,%03D",X,Y); X=X+SX*8:Y=Y-SY*8 IF X<0 THEN X=0 IF X>(400-16)THEN X=400-16 IF Y<0 THEN Y=0 IF Y>(240-16)THEN Y=240-16 SPOFS 1,X,Y WEND