When I turn the hot water tap on the flow is normal for about 1 min, then it starts to slow down and eventually stops. I turn the tap off and then back on and it does the same again.

Good morning Andy, While you didn't say, I'm going to assume this valve, (kitchen, tub and shower or bathroom lavatory?? ) has two handles and not one. Sounds like you have something in your hot water supply line. A small pebble, a tiny piece of solder that's broken loose or it could be a washer that's came apart. When you first turn on the water the flow's is OK. Then the obstruction works its way up the line and blocks the flow. When you turn off the valve it falls back on down the supply until you draw on it again and the whole thing starts over. Again, assuming a two handle valve, you will have to shut off the hot water at the shut off valve located in the cabinet. Remove the handle. There will be a nut on the stem and washer assembly, this called the bonnet packing nut. Remove it and back the stem and washer assembly out counterclockwise. Check the washer If it's worn or broken, replace it. Also check the seat for nicks or rough spots. Now place a pan over the open valve body to reduce splash and open up the hot water shut off valve. You are flushing out the supply line. Reassemble the valve and test out. Hope this helps and thank you for using AskMe. Tom