How can I fix my Baxi Combi 105e?

The boiler (about 7 years old) loses pressure quite regularly and I now know how to increase this, but I am having to to it more often. The shower can be lovely and hot one minute and then goes stone cold. It used to get hot again after a few seconds which I understand is all to do with the pressure, however, now I am flicking the heating on in the mornings this seems to affect the hot water flow. It will be hot, goes stone cold after a couple of minutes and then does not get hot again - pilot light goes out. Why? It is not so bad if I don't put the heating on first but still occurs. My plumber said he cannot fix it whilst the boiler is working! (Yes, I am going to change my plumber.) I then have to keep flicking the ignition switch backwards and forwards to get the pilot to light again which is getting more and more difficult to do and then the same problem occurs. Any ideas?