Radiators not all working all the time???

We have a problem with our heating, we think it started after we bled some radiators and took one off to wash threw. The rads downstairs are not getting hot, the rads nearest to the boiler appear to get hotter. We have hot a plumber out four times and he has not found the problem. He has bled the rads, balanced the rads, changed the pump, changed loads aof thermo valves on the rads, drained the system and refilled (never did a power wash as the rads were not really that dirty and never saw any sludge). We have since tried turning the honeywell motorised head valves onto manual and the heating kicks in for a couple of minutes then switches back toi auto again, what are the possible causes of this?

Jue
January 2011