I also have a 1998 Mercury mystique I had the same problem with it!! My car would loose power and speed as well!! Have you had your transmission serviced lately? Get it done I really am serious!!! I had to replace the wiring harness and the transmission as well!! Please go to the nearest ford motor company and spend the little bit to have it put on the machine!! I could save you to have to replace major parts. Do not Drive your car anymore until you get it fixed!!!