Ariston microcombi 23 MFFI?

Have a problem with heating. 2 rad's upstairs hot and working OK, but down stairs no heat rad's remain cold,I have bled all rad's.
The hot water is OK but can be intermitent i.e. cold then hot.
Did notice that feed pipe to one of the rad's down stairs was starting to warm up but not the rad so could it be the TR valve on radiator.
any one have any sugestons.
Cheers pauljag.

I forgot to mention that when I looked at the boiler initialy it had shut down i.e. red light showing ,so I went through the reset instructions.