Strange program crash

I've been trying to troubleshoot a mysterious problem for a few days now, and I hope someone here can give me some ideas.

We are using UV3 and C51 (ver 7.50) to develop an application for Cypress FX2 LP which is a 8051 variant with 16KB RAM (which may be shared for code and data).

For applications of up to about 11KB in code size, everything works fine. When the program exceeds 11KB, strange problems pop up. Calling some (valid!) functions trigger a system reset. Adding a line to a function that no one ever calls, and the system goes into infinite reboot loop. Adding a dummy file with some uncalled functions, same thing happens. Adding a redundant function call that does nothing and everything goes back no normal, until the next time some other line changes. To make a long story short - total chaos! I can't understand the logic of any of this, and it happens only beyond a certain program size. The program compiles and links with no errors (except "uncalled segment" warning if 'dummy' uncalled functions are inserted).

The problem is very distrurbing and very chaotic - the program may run fine up to a certain point, then reboots with no apparent cause at all. Comment out an unused function and all goes well. Another thing I noticed - if I tell the linker to move the program in memory, the fault point shifts randomly to someplace else.

We've ruled out a H/W problem or a defective CPU. Has anyone ever encountered something so odd, and can you give me any ideas on how to troubleshoot?

Thanks in advance,

Ran Shalgi
Siano Mobile Silicon