Recently I have been having trouble getting good water pressure to come out of the shower head (for hot water only). I get good water pressure (hot and cold) out of the tub spout. Also, I am able to get good water pressure (hot and cold) out of the bathroom sink faucet. The shower has a diverter tub spout. I am not sure the exact model (since I don't own the townshouse), but it appears to have the shape of the Moen 3931 Diverter Spout with the handle of an American Stanard Slip-On 4" Diverter Tub Spout. When I pull the lever up to activate the shower, I notice a somewhat steady stream of water (slightly more than just a trickle) of water still coming out of the tub spout. Also, it seems to take a few minutes until I hear the pipes make a weird sound and I get full pressure out of the shower head.

Tonight was a little bit different because it took me about 10 minutes of playing with I finally got somewhat decent water pressure to take a shower. When I went to turn the handle (cycles counter clockwise from off-cold-hot) I noticed the water pressure jump up to normal when I turned the handle through the cold portion. Also, I noticed the water sounded "airy" or as if there was a leak somewhere while I took the shower.

What could be causing to my shower to have trouble maintaining pressure when the water is hot and able to maintain normal pressure when the water is cold?