(and I was one of those that had communication problems with the newer RH version - I dont recommend updating past 1.0.6 at this point. )

- 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.

Another random thought...have you upgraded the RH recently?  Some users have experienced random connection issues with the newer RH Versions.  If so, you may consider downgrading to a prior version.

Just a note: I set up my MakerGear M2 on the same computer on the same circuit and it has been running without a hitch since I connected it three days ago (running prints nearly 24/7). The main differences, I suppose, are that the M2 is using Pronterface instead of Repetier with the SD2 and the M2 has a stock power supply from the manufacturer instead of a modified PC power supply, plus it is only 3 days old now. :-)

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

- 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!

I've been running this SD2 nearly every day for over 2 years. I upgraded to the heated bed about 3 months after I started using the SD2, and that's when I added the larger power supply. After the problems started happening, I tried running 3 prints with a high volume fan pointed at the mainboard. The problems persisted.

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?

I didn't see any power rating on the board.

I do a lot of work with 5V, so I got confused. I may still be confused. Does 12V@12A sound better? If not, what would you recommend?

I checked all the connections. Any ones that were suspect, I removed the connectors and soldered the wires together and insulated with heat shrink tubing (the extruder heater). For the power supply, I'm using an overkill PC power supply (5V@15A)

I'm thinking there is a problem with the MCU on the mainboard. I've been running this SD2 nearly every day for over 2 years. I upgraded to the heated bed about 3 months after I started using the SD2, and that's when I added the larger power supply. After the problems started happening, I tried running 3 prints with a high volume fan pointed at the mainboard. The problems persisted.

I've been monitoring the printer when it stops working. There are no signs of any loose connections. If I wiggle the wires at the different connectors while it is printing, it doesn't cause any problems, except on the extruder. That's why I bypassed that connector with a solid connection.

Any ideas?