I think I've found the cause of my PI sometimes crashing, and I therefore withdraw my accusations that the ModMyPI case was making it overheat!
When I bought my PI, I already knew that it could be a bit picky about the PSU, and needed a good quality one, so I ordered one suggested for it that I think is rated at 3Amps, even though the PI by itself needs only around 750 - 800mA I think.
Anyway, one of the reasons for crashing can be low voltage. I finally got around to checking my PI, by putting a meter across TP1 and TP2 on the PI. This should be a fraction over 5 volts for stable operation. Mine was struggling to stay there, at one point I saw 4.5 volt as it was booting up. And this despite me not having anything plugged into the USB ports to draw extra current - no keyboard, no mouse, no disk.
So, think I'm going to have to find a better quality PSU! And put my PI back in it's case
Ian