Solved Bed stops heating at around 64C

Are there any common reasons why the bed on an R1 would reach a certain temp then drop in temp?

When i start my bed heating, it will reach around 64C before the temp will just start to fall. I had a look at the terminal output to see if there is any error, and there isnt. It just stops heating.

I had to replace the RAMPS board after i blew a FET on the weekend, but after replacing, i was able to print ok. Once or twice it did this fail to heat thing, but i found turning it all off and on, resolved it. Now, I cant heat the bed for a print at all.

I have checked and re-seated the all the connectors on the RAMPS board, and the fan under it blowing fine. I have found if i close Mattercontrol and re-open it, the bed will start to heat again, but reaches around the same temp before cooling again.

I searched the forum and found a few similar posts and they all suggested the connectors, but i've checked and re-seated everything 3 times.

Suggestions?

 

After flipping the printer, removing and reseating all the connectors, unscrewing and rescrewing all the screwclamped wires, i noticed one of the wires to the heated bed wasnt in the best shape. A couple of the strands in the wire had broken and it was down to just 2 strands. I cut it off, freshly stripped it, and re-screwed it, and lo and behold, the bed heated to 90c!

Great i thought... and started a print... only to have the bed temp start to fall while printing.

One thing i did notice while i had it upside down, the red RAMPS board that came with the printer says its a 1.4A. The one i replaced it with is a 1.4. Is there any version difference i should be aware of?
This is the one i used:
http://www.sainsmart.com/sainsmart-...amps-1-4-for-arduino-reprap-mendel-prusa.html

 

Few extra details, i have noticed there is a LED by the FET for the bed that lights while heating. But goes out when it stops.
If i then set the bed in software to 0c, then tell it to heat, it will start heating and the LED comes on, but moments later, goes out and starts to cool again.

Could it just be a bad bed FET? I dont have another spare RAMPS board, but i do have 4 spare FETs.
I was going to replace the FET on the original board but unfortunately while desoldering and removing it it took the traces off the board with the FET pins

 

I checked, re-checked, and checked again all the cabling. Re-seated/tightened all connectors, re-stripped and re-tinned all screw clamped wires to no avail.

Getting annoyed and suspecting the FET, i pulled the whole thing apart and on close inspection noticed that one of the large yellow square capacitors had a crack on the side closest to the FET. Was hard to see when still mounted in the printer. I desoldered the one from the other failed RAMPS board and swapped it out. I had some spare FETs as well and figured "what the hell might as well while i'm here" and replaced the bed FET, and took the opportunity to add a small heatsink to it.

Re-assembled, and presto! Heated!

Did have to fix one of the wires for the x axis end stops as one of the wire joints broke while messing about, but after all that, was printing again.

Those bloody bed FET's, root of all evil!