What could be causing low hot water pressure to a sink?
Disconnect the hot feed from the faucet and purge it into a bucket. Then, (re-attach it and disconnect it from the valve) put the bucket under the sink (to catch the flow from the loose line) and run the faucet in the middle (or both hot and cold). 

This will purge the hot side of the facet, backwards (and make a mess if you're not careful), in case there's crud in the tap now, too. Open and close ALL valves repeatedly to clear them while you purge.