radiators cold upstairs, hot down?

Hi - I have 3 radiators out of 4 upstairs that are cold. Downstairs are hot. Ive changed the valves, bled them all and had the system flushed. The leg is hot on one of the broken ones but it still fails to heat the raidators.

When I turn off all valves downstairs my combi shuts down after 20secs after the temp hits max.. any1 know what could be wrong. already spent £500 and I still have cold rads :( any help is appreciated. ta

mark