Visual Foxpro 6 crashes

I am trying to write a program for my company and for the last 12 months have been plagued by crashes in VFP6, all more-or-less exactly the same, even on different PCs, and amounting to this:
VFP6 caused an invalid page fault in module VFP.exe at 0167:00423b37, in the EIP register. This is in IDE mode while developing, not while running the compiled program. Crashes happen when a wide variety of actions are carried out. FoxPro runs OK after restarting until next crash. I only use forms and prgfiles, nothing clever or glamorous. Any ideas, oh Great Ones?