When running hot tap at first no problem with hot water but water slowly cools down. boiler is a combi boiler?

I think it's to do with your water pressure, I was getting this (hot then cold then HOT! lol) for a while until I fixed the pressure (should be a knob under your boiler). It helped, but now it doesnt happen so much when the heating is on anyway, sometimes when the system hasn't been on for a while.