ahh this problem, i have encountered this every game. i did solve it once. this was caused by the bullets not getting destroyed outside the playfield, although i did have a command for it to destroy.
i solved this by having extra commands like
~when bullet leaves play area = destroy
~when bullet outsider play area = destroy
and so on.
doesn't seem the best of bug fixing but it worked for me
Destroy bullet when it reaches -6 pixels from window fixes your problems spiderhead...one simple line for small objects.
Anyway, one problem with the paste into background command - as soon as any part of the pasted background leaves the window, it vanishes. This is annoying as it can chop bullets, ladders, etc. in half when pasted into background.