Potterton Puma 80 heating problem?

Hi there i wonder if anyone can help i have two probs.
Firstly heating switches off every 30 secs even on constant.
Is this the wax diverter valve problem?
Also all rads downstairs are stone cold.
Have change the pump still the same.
Could it be down to sludge etc in the pipework.
Boiler is in upstairs bathroom cupboard if that helps.
Many thanks for any advice given.
Paul