Boiler Hot water intermittently losing temp, Heating seems fine?

The title explains the main issue, for some reason the hot water drops often every 20 secs or so, sometimes it doesn't recognise that the hot is even running right away but always seems to click in within 45 secs?
The shower runs direct from the boiler also and I have to get a 30 sec to a min shower then turn it off and wait 30 secs turn back on and hope I may get another 30-60 secs.
I believe there is loose wire somewhere as shutting the flap on the front anything more than softly sees the boiler power down before firing back up again but I believe this isn't the issue.
I don't know much at all about the workings of a boiler but we did change a pressure valve a while back, I forget what the part is called but we had to remove the casing and then open the boiler up, the valve sits on top of the upright pipe on the right of the boiler and was a pain to fit as even with two wrenches on it there was a third section that wanted to turn lose.
It seems to me that the problem is with a sensor that either tells the boiler when the hot is running but more likely a thermostat which maybe telling the boiler it needs to shut off every time it gets to temp?
The final point of note is that the boiler hs always lost pressure but in 5 years there has been no sign of a leak, the funny thing is that it used to lose pressure every 5 days to 2 weeks but after 3 years it stabilised and seemed much better but lately its losing pressure to a similar degree as before, I wondered about the valve we fitted which was over a year ago but there is no sign of any leak.
The central heating seems fine.
The only thing that has always bothered me is why did the plumbers who fit it leave the flexi pipe in place, they told me if i ever need to add pressure I just need to turn the two handles but I have since been told that the pipe should not have been left although with the average rate that the boiler loses pressure i'd be lost without it! the pipe actually leaks a fair bit of water from the top handle when turned which it never used to do.
Any advice/suggestions are greatly appreciated