I had this problem in my converted freezer also. I found that the tempeture varied greatly with the distance from the bottom of the freezer. Placing a small fan on the compressor hump equalized the temperature from top to bottom, so now my beer flows nicely as soon as the faucet cools down (about a third glass of foam, subsiquent pours don't have this problem).One other thing it could be is a dirty faucet, bit of hops stuck in there.
I thought it also may be the temp but I'm reading about 38F on top of my keg and you're right about the temp being colder at the bottom, about 32F from what I can tell. The fan sounds like the next step though. What type of fan are you using? Does it run all the time and how do you have it wired in?