1996 ford escort runs good cold wont accelerate when warm?

my 1996 ford escort is an otherwise fine car except it has developed a problem, when i first start it up it runs like a top but after about 10 minutes it warms up, and when im at an intersection it accelerates extremely slowly as if it has no horse power at all until i get it up to 15-20 mph then it begins to pick up speed. what could this possibly be? (it idles fine)