Pressure Drop?

I have a Grant Combi 90 Mk II boiler and approx 5 years ago had a tiger loop (GOK GS2000) fitted internally next to the boiler as my then service man said the original one was illegal to have inside. Ever since then I seem to have had problems despite getting boiler religiously serviced each year (installed 9 years ago). The tank is lower than the boiler. The main problem is the system pressure guage which drops, goes up, drops again etc etc. Each time I get told I have a leak in the system. However I have recently had another boiler repair attempt at shutting down each radiator in turn to isolate the leak but none was found. If there is a leak it does not explain why one day the guage goes down from say 1.5 to zero in about 30 minutes and when water put back in system (not necessarily very much - ie not the whole system) the gauge takes anything from a few weeks to a day to go down. My latest repair guy replaced the pvr (?) but I believe that was done last year. He also indicated the expansion vessel (had about 7 of those now) was not big enough for the property and that I should consult a plumber to install an extra external one which I did. His immediate and initial reaction was that the expansion vessel should be big enough and checked size of radiators etc then informed me I had a leak. Explained to him the aforementioned and he was mystified and said he would come back next week but I won't hold my breath. The reserve of hot water never seems to be there and it takes a while to kick in and some days my upstairs shower is pretty cold to say the least. If I run my water reasonably slowly - as I used to - it runs cold. I do understand that if this flow is too slow the boiler is not really 'seeing it' but this does not explain why it used to do it!. I know this is a very long-winded question but I am at my wits end with working my way through the local boiler maintenance teams and plumbers - so after a service just 3 days ago the system pressure gauge was at zero by the end of the day. Have kept a couple of days worth of time/pressure/heating on or off etc and this has ranged from zero then over 2.5 once heating put on. Is there ANYONE out there who can help me?