IF it was/is the PSU, I'd suggest testing the voltage rails. Also, what PSU were you using? Better to be safe then have a broken PSU break another mobo.

Also I read your ecs board uses an nvidia chipset. Those don't have a good reputation anymore. Same goes for ecs imo.

PSU is a Blue Storm II, which seems to be a middle ranking brand/ model. It's 550w I think. how do i check the rails?

The reason I mentioned the PSU is that this was the PSU I used with my asrock/X2 system once upon a time; that developed a reboot/freeze problem that I thought I had solved[there was a stick of bad ram], though there were still some very intermittent bsods i think but i put those down to messed up nvidia driver installation at that time. Since shifting that to a different box/psu it's been rock solid, but i also changed hd's and dvd drives.

It kind of worries me that although the x4 system with the ecs ran fine albeit sluggish for some months, it eventually developed symptoms that paralleled those of the x2/asrock system at one point.

Looking back I also had a 6800gt fail, but I don't think I used that in the ecs board (and it had been intermittently failing for a year or two). Probably irrelevant.

Capacitors on the motherboard, most likely. ECS tends to use cheap caps, and cheap caps tend to go bad, with the same symptoms you're experiencing.

Nvidia doesn't have a good reputation for chipsets, but the 6100/6150 was one of the few they actually got right. I wouldn't say it's any less reliable than similar chipsets from AMD or Intel.

I recall now that I checked the psu voltages with a voltmeter when I started puylling the system apart, and all were ok. I also remember hwmonitor [on the ecs system] reported the 12v was giving 5v when the ecs board was dying but I assumed it was some software/sensor incompatibility.

Also the psu has been more heavily loaded for the last few weeks in another system. So [despite the reservations that prompted some of my post] I think I've convinced myself the psu is fine.

Bad caps eh? I didn't realise that might be an issue with modern boards, though come to think of it the gigabyte box makes great play of the tough capacitors they used for my new board.

It's not nearly as common of an issue as it was ~5 years ago, but yeah, it does still come up from time to time, especially on cheap boards. If your board is similar to the Gateway version (a couple of which I've had in for repair), it's probably the 1800uf 6.3V caps around the CPU.

man ECS is a shoddy brand I have a pentium board so far from them that runs perfectly for the moment but tons of them are CRAP

you seem to think that instabilities are due to chipsets. pentium via and sis chipsets yes are slow... but they're really stable nevertheless... stable as intel ones