Mixer shower temp problem?

I have a Alpha CB28X combi boiler and a mixer shower attachment.When using the shower the temperature of the water is not constant, gets hot then goes tempid and this happens all the time the shower is in use.
I have stripped,cleaned and examined the mixer shower attachment which seems okay. The shower has worked with out a problem for a long time but this water temp problems is happening everytime we shower.
I can only assume there must be a fault some where in the combi boiler, there has been slight variations in temp of the other water outlets , hot taps.
Has any one else had and dealt with this problem.
Thank you guys.