hot water problem with my baxi combi 105e?

i have a baxi 105e the heating and hot water was not working. On investigation found that the 10mm pipes were blocked on the heating tried power flushing it but ended up re-piping it as all rads bar 2 were totally blocked. i also removed the heat exchanger and hot water plate exchanger cleaned them then powerflushed the system and inserted more x400 to stay in for 4 weeks. The heating is now working but the problem with the hot water still exists the boiler fires when hot water is demanded but keeps heating the water until the boiler temp goes to 80C and shuts off via the high limit stat then cools and does the same again. I have changed the hot water temp sensor but did not work, the book says the gas valve or the PCB but the water is progressivly getting better so i think it might be the x400 in there cleaning something. Is there anything that i could clean to get it back on cheers Ray P.S i am corgi registered but have limited knowledge of service as i mostly do installation