I've used Test Program from Aminet this afternoon and it consistently picks up errors in FastRam, so a relief it's not something on the main board and hopefully it can be resolved by cleaning the SIMM contacts (at worst replacing the SIMM).
Update: I think it's software related. If I boot from the TestProg floppy there are no errors. If I boot from HD (no power cycle, just reset) run the app from Floppy in WB, I get errors in the last MB of the FastRam test. Another reset and boot from Floppy again and the errors go away. I'm just doing a soak test now.