Hot End Temp only 135C - PID (no replies)

I know this is yet another Hot End temp problem but I can't find the solution, so here I am...

I have recently upgraded my electronics to Ramps 1.3 and Marlin firmware (from Gen3) and I am using Pronterface. The problem I have is that I can not get the Hot End temp to go above 135c. It heats up steadily and I get a temp reading which then reaches 135c, but then it drops off. I have a separate thermocouple for double checking temp, however, and this continues to rise despite the termistor reading dropping off.

Troubleshooting I have tried:

I initially changed the PID values to the ones which worked on my previous setup. - No Change

Then I changed the PID settings a few times, including setting 'I' and "D" to zero and then upping "P" - No change

So then I tried to use Bang Bang. Still the same response

With all these attempts I have tried setting the temp at different values, but it still only ever gets to 135c and drops off.

One thing I want to say about the PID Debug messages in Pronterface is that it gives an input value feedback but the pTerm, iTerm, dTerm values remain at 0.00. is this normal? I was expecting some activity there..?

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What I find strange is that I know the hot end/ power supply (ATX 300w) worked in my previous setup, but no matter what settings I try I get the same response. The rest of the printer seams to work properly (all axis steppers move and optos work) but I've yet to print anything on the new setup.

Any help on this would be gratefully received.

Cheers

Graeme