Maximal size for a compiled program

[Week of the agents]

Hi, I am developing game with ZX Basic for a Spectrum 48k and when total size of compiled code approach up to 35-36K shit happens. Program can hang on or some pixels on screen are highlighted or INKEY stops to working.
Is there some limit?

I load code at address 24576 (24K) so I thought I have 40K room for free.