poterton performa 24 how to fix?

i have a poterton performa 24. when i put the heating on it stays on until upto temp then it fires up allmost contantley on and off with a few second between . it has now started to switch on as heating even when i turn it just to hot water only the heatin thurmostat doesnt seem to have any affect on the temp of radiators either any advice would be greatfull