Thermo run away

Hi, for a whole year since I built the Prusa i3, I had no major problem. But recently it has been acting very strangely. When I started to print for an hour or so, it would randomly stop.Ts is especially annoying when an object is printed 4-5 hours down the road. I thought it was a problem on the RAMPS board, so replaced it with a brand new one. Added a fan which blow directly at the RAMP board. The problem persists. I commented out the over heat section in the Marlin firmware. Now when it stops, it will give me the Thermo run away error.I have checked all thermistors. And when I install the new RAMPS board, I replaced all wires with new ones and made sure all connections are secured.I also connected the USB to a different laptop to rule out the USB. I can't think of anything else that could cause the problem. Any suggestions?

Edited 3 time(s). Last edit at 01/09/2016 07:52PM by rich1812.

Have you used a surge suppressor?
BSCP(BullSh!!ComputerProblems) have plagued me my whole 40 year career.
For instance:
If you live in a cold country or are printing in the winter and furnace kick[KICKS} on there will be a spike in the mains. Just sayin.
Or air condition in hot climate.

I know people grumble about PC power supplies. But if you wire all the yellows together and wire all the blacks together and wire all the reds together on a surge protected power supply you get a really decent power system. 120VAC @ 32 AMPs. Excuse em wuah?

Edited 5 time(s). Last edit at 01/09/2016 10:04PM by Giantkiller.

Thermal runaway is triggered by the hotend or bed not heating up in a given time interval. This is meant to be a precaution against thermistors that fall out of their holes. If the heaters cannot detect a temperature change they will keep on heating up until thing start to melt and catch everything on fire.

So, check your thermistors and heaters. Make sure they are wired correctly and that temperature swings are not too great. Compare the values to that of known values like room temperature. If they are inconsistent replace them.

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Prusa i3 Rework - Ramps 1.4 - E3d Lite6 - Full Graphic LCD Controller

I had this issue with a heater cartridge that went bad. Just replaced it and never had a problem since.

This sounds like and issue my printer suffers from. If its like mine, My thermistor is messed up. If this occurs I can look at Repitier and see that my temp has run away (maxed out) or is at zero. If I wiggle my thermistor wire at the hot end I can watch and see my temp reading return to normal. If this is the case you just need to replace that part.

Appreciate all the replies.
As it turned out, it was caused by the loose wire from the heat bed connection. Perhaps it's because of the vibrations during printing. once I tighten it up, it works perfectly