How-to guide for changing a blown mainboard on an SD2?

All very true, you board isn't your problem.  Sounds like you are getting power interruption that is creating interference with the USB connection.  Check your PSU internally and any and all power connections.  Fans and the like, a loose wire will cause your symptoms.

"I'm using an overkill PC power supply (5V@15A)"
Is that a typo?

No sarcasm at all, just wanted to verify.  At 12 amps you're not exactly over feeding the machine but it should function fine.
What's the wattage rating of the upgraded heat bed?

Just to clarify, are you saying that you upgraded the heat pad and PSU 3 months after you started using the unit?  If so, then it was running fine with those upgrades for roughly 21 months...

So, assuming the random stops have been a recent occurrence, have you installed or changed anything that would place a sudden electrical draw on the 115V house circuitry, like a refrigerator, fluorescent lighting, etc?

when did you last clean and lube all mechanical parts of the Printer that move? stiff movement can over heat the stepper controls fast during lots of quick movement during printing making them reset, how long does it shut down when it does? another thing that can make printer "pause" mid print is using the same PC your printing through for brousing the web streaming audio/video or playing games, as they all rob band width from the USB cable connecting printer to RH on the PC. happened to me lots so I ban myself from such things while printing.

Thanks for the responses. Here's the answer to the questions:

- Yes, the printer has been running fine for approx. 23 months since I upgraded to the heated bed and 12V@12A power supply. No, nothing electrical has changed. This is on its own isolated circuit and plugged into a surge strip. But that's a good idea. I'll try it on a different circuit to see if anything is different.

- Yes, this is on a surge strip.

- I last cleaned and lubed all the mechanical parts about two weeks before the problems started with high quality bearing and bushing grease (Phil Wood). When the problems started, I cleaned and lubed all the mechanical parts again. But the failures will happen as quick as a few minutes into a print where the steppers are moving slowly on the first layer. Other times, the printer will print fine for 2 hours or more and then stop.

- The PC used for running the SD2 via Repetier is dedicated to only that. I don't use it for anything else. But I'll try a different computer just to be sure.

- I'll verify the voltage at the PSU with the heaters off and on. That's a great idea to check for pulling too much current.

I'll let you know the results of my testing. Thanks again!

so that PC PSU has multi tap outputs and cooling fan/s right? those have been known to fail without warning either part or all power outs and fans which would overheat randomly even if just dirty. keep a can of air nearby for that depending on ambient dust/hair conditions. just a thought. also there are some bugs and vermin that like to eat wires if they can get into enclosures. had to replace lots of electronics due to that when I done Appliance service calls at homes etc. dont matter how clean winter brings em in. also check any toggle switch contacts on that PSU

Thanks for the additional ideas!

- The power supply is clean and has a cooling fan. All the connections seem to be solid. But I hope to test it today to see if there is any voltage loss under heavy load.

- I'm using the same version of RH as I was a year ago.