Temperature on Mira Excel B running hot and then cold?

Mira Excel B shower working off Combi Boiler, water pressure is good, but temp keeps going up and down. Have flushed boiler with Sentinel deposit remover to make sure it is clean, but water temp is still not constant, would be glad of any advice to rectify the problem, hot tap in kitchen sink and bathroom basin seem to stay constant.
Thanks, Keith.