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

