I have a 98.5 SVT that did the same thing. It will get probably worse, and the engine will start racing in idle. Mine was hitting 4000 rpm. I took it in and the dealer replaced the idle air speed solenoid/sensor (something like that). They said that is a common problem with Fords from 96-99. My friend has 97 SHO, and he has had the same problems.
Have your dealer check that part out. It costs about $170 if your warranty has expired.