Hello,
I recently installed a new Price Pfister single handle Kitchen Faucet. (July 2008)
It has been working fine, no leaks and no problems until just the other day.
When the hot water was running, it slowed to a trickle.
I diconnected the line from the valve, and checked that the valve is functioning properly.
There is a strong feed from the valve.
The cold water is working fine, but the hot water is just trickling.
How can I determine where the problem is?

Sounds like you have some trash blocking the hot water inlet ports of the faucet cartridge. Shut thewater off and open up the faucet. Remove the cartridge abd check and clean the inlet ports. Since you have already checked the supply and found it clear open the hot water angle stop to flush out the valve body. Reassemble and test. Better now? Good luck and thanks for rating my answer. Tom