All the delivery problems I have run across have been a result of sediment in the pipe. But your symptoms seem atypical. Copper pipe usually does not build up a lot of sediment and I don't understand why the flow decreases as it gets hotter. However, sediment problems do show up in hot water pipes more so than cold water pipes. I'd begin by taking the faucet apart and looking inside.

If you turn the faucet off when it is slow and hot, leave it off for a few seconds and then turn it back on again, is the problem still there? You could try turning off the hot water heater and then running the faucet to determine if the problem is caused by the heat or the flow.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13