I have similar boiler with similar fault. During summer when mains water is warmer, shower taps are mixed more towards 'cold', so less hot water is being drawn off. Pattern is:
1) Turn shower on full hot to fill pipes up
2) Mix cooler when pipes filled
3) Runs cold. Stand aside and repeat.
Also, presumably boiler has to put less heat in to warm the water up if this has anything to do with it.
This isn't a new problem. Service people are happy to replace parts at vast expense - and very often on a 'suck it and see' basis. i.e. "This will probably work, but if it doesn't give us a call...!"
I appreciate things get corroded and dirty and need cleaning / replacing now and again (pink/brown crud from the water in our area, Tyneside) but I would appreciate some clues / priorities rather than experimenting for myself please.
Thanks in anticipation!

Steve Milner, June 2009