Vokera Linea 28 Pressure and Hot Water Problems

[yorksmags]

Had a problem with my 7 year old combi not holding pressure and following advice from Vokera re-pressurised my expansion vessel. (This was after the heating engineer who called said it needed a new boiler without even taking off the boiler cover.) Heating runs fine now.
However there is still a slight drip from the external drain on the boiler and re-pressuring the EV lasts around 7 days before the pressure gauge drops down to low again.
No water comes out of the EV valve when pressed, just air but there must still be something wrong for the leak and drip to continue?
Also when running hot water off, the boiler runs for a couple of minutes, then cuts out for 10 secs and then fires up again-during a shower you have to be quick to dodge the cold water!(Thermister problem?)
Not wishing to do any more on the boiler DIY but just need to be sure when calling another HE I get a sensible diagnosis and have some suggestions myself.
Any advice gratefully received please?