A common scenario.
1.) Check the seating of heatsink, ensure stability and it is on the CPU. (You have done this, no doubt.)
2.) Ensure power supply generates enough power, and that it's functioning. (You put in a solid 400Watt, so I think this is good too.)
3.) Check cable connections for connectivity and for anything burnt. (It's what Indian tech support tells us to do.)
4.) Ensure nothing is shorting the board on the backside and such. (Screws could fall down there and no one would know.)
Considering your situation, I'd say four years is a long time of operation, but that instigates you maintained the computer well, and that its parts were stable in general. Motherboards are troublesome, as it is impossible to tell damage besides a lack of activity. Sometimes we see apparent bulging capacitors as you mentioned, but most of these semiconductors go down silently. Power is often the issue, and if the motherboard receives an energy spike, the current is sure to do some sort of damage.