Why would the RPM go up and down in a 97 Mercury Mystique?

This happened to me today on a major highway. The Ford garage told me it is a transmission problem. Check transmission problem solutions here in this blog. Solenoids, etc. It means your transmission is slipping, that happens in a lot of Mercury mystiques. not all true there is a throttle position sensor or T.P.S. on your throttlebody try that its way cheaper than transmission