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It seems to be the season for plumbing related queries so I'll add mine :p
My kitchen hot water tap sometimes has a really low pressure and I can't figure out why. I only have two hot water taps in my house and the other one (in the bathroom) never has the problem. I've also tried the bathroom tap when the kitchen tap has low pressure and the bathroom tap pressure is still good.
When the issue occurs it usually starts about 1/3 to 1/2 normal pressure and gradually decreases (over about 10 seconds) until it's not much more than a dribble (it usually runs cold at this point to but I think that's just because the demand isn't enough to kick my combi boiler into hot water heating mode?).
Sometimes when this happens if I turn the tap on and off a few times in quick succession the normal decent pressure comes back, sometimes this doesn't work though (but if I try it again a couple of hours later it's usually back to working properly).
I can only think it's some sort of air lock or physical blockage but from what I've read you shouldn't be able to get an air lock with a combi boiler (I might be mistaken on this) and I wouldn't have thought a physical blockage would be so intermittent. The tap doesn't drip at all and seems to turn the same amount each time I use it so I don't think it's washer related?
When the issue occurs it usually starts about 1/3 to 1/2 normal pressure and gradually decreases (over about 10 seconds) until it’s not much more than a dribble (it usually runs cold at this point to but I think that’s just because the demand isn’t enough to kick my combi boiler into hot water heating mode?).
One of my mixers does this, I think you right about the combi thing. Replacing it with a higher flow mixer might sort it.
Sounds daft but have you checked the valve in the tap? I have had a similar issue and it was the tap cartridge thing. Took minutes to replace and was cheap.
I only know this as I employ a few plumbers and asked them to come and have a look, they all answered the same thing.
I was going down the same road as you and considering a complete house re-pipe.
My Dad had this issue in a few taps at his house. little bits of flux, gunk, dislodged scale get pushed around the system until they end up at the taps, where they can't get any further.
It's probably a bit of stuff that falls down to a low point/elbow when there's no flow, but when you turn the tap on, gets pushed up to the olive/washer at the tap tail, where it blocks the flow. turn the tap off, no pressure, it falls back down.
I took the tails off on the problem taps at his and poked around with a little pick. Needed to turn isolator valve to flow again on some of them to push the gunk out of the end of the pipe if I couldn't get to it otherwise. Bit of hose and a plastic basin recommended!
I had the same issue. Combi boiler, bathroom tap works lovely, kitchen tap was behaving as you describe.
The tap washer was the issue. Easy to fix as described above and costs pence from local wickes. Quality of life has been improved immensely.
Flow straightener on the tap spout should be the first thing to check if it has one. Very common to be blocked very simple to fix. Sounds like a good issue rather than pressure too.
My bet is on a collapsed flexi hose to the tap. Is it on flexis?
Hard water area
Coulf be calcium carbonate build up, but i tjink this sounds like heat doing funny things to the washer
Swap the HW cartridge with the CW cartridge - if the issue moves to the cold tap you have your answer.
May or may not be related but something I didn't know was that there are high pressure and low pressure rated taps - & from experience high pressure taps on a low pressure system exhibit the symptoms you describe - although being a combi boiler you should be getting high pressure 🤔