18Fs using PBP have a 27 level stack. That is a lot of GOSUBs.

Whatever you do - keep track of your GOSUBs. Don't ever GOSUB to a routine and then use
GOTO to get "back".

Every time you GOSUB, you put an address on the stack. Every time you RETURN, you pop that address and resume execution there. If you push addresses there without pop-ing them off, you will eventually get a stack overflow which causes a processor reset.