The PC stops immediately when using a new PSU, yet it functions properly with an older PSU?
The PC stops immediately when using a new PSU, yet it functions properly with an older PSU?
It may not be the Montech PSU, since it is of high quality. But there are lemons even among the best and you'll never know. After all, there is a reason why brand new hardware comes with a warranty.
I'd also look towards PC repair shop and PC diagnostics. Since your old FSP PSU is such a crap unit, that i would not trust hooking it back to the PC.
Could be, that your MoBo died (be it old age, caps giving out; or ESD or short circuit). Impossible to tell.