OK, the problem is really apparent tonight. I've been un-LHA'ing so perhaps the extra activity has caused things to heat up a bit. However, it's been running fine for the past hour - it's only now starting doing this. It also happens right after a reboot.
Take a look at this:-
http://www.youtube.com/watch?v=am73Zsn6ntA
This is now running in high res DBLPAL mode, and the same problem occurs with and without the accelerator connected.
Definitely seems like it's heat related - but should it be doing this with just a 2.5" internal hard disk? Is DBLPAL enough in itself to heat the chipset this much?
Or - am I looking at a bigger hardware issue here?
Opinions appreciated 
I'd suggest you disconnect everything from your Amiga including your ram expansion/turbo board. Then boot up without the startup sequence and wait and see if anything happens.
If all goes ok, then one by one add back your peripherals and wait for the problems to occur this should help to narrow down things a bit.
