95 Mystique surging

Hi Folks, My often problematic 95 cyl Merury Mystique surprised me today with a new problem. Cruise control failed on the highway. When I came to a red light, the engine started to surge badly. This continued to happen after the engine idled for about 10 to 15 seconds. When I gave it gas it seemed fine. At idle I have to put it in park to keep it from trying to jump forward. Any ideas? Thanks, Bob