I also have a 98 SVT, and am having the exact same problem. It seems to be getting worse with the humidity. Have you done any modifications lately? When I first start it up, the RPM's shoot up to about 3 grand then bounce down and back up until it finally cuts out. I posted this concern but haven't received any replys. let me know what you find out.
Recently I:
Removed the resonator
Added intake ( about a month ago)
Changed the oil
Used fuel injector cleaner
Changed out fuel filter
If you did anything similar let me know, I'm not comfortable driving the car right now because of this problem, so let me know if your having any trouble when driving. Mine seems to be running really rich.