What if a 1995 Mercury Mystique transmission has suddenly started shifting at higher than normal RPMs and clunking into gear but doesn't seem to be slipping?

Change the transmission fluid and filter. If you see no improvement, take it to a transmission shop. We had a similar problem, and it ended up being a sensor in the transmission. I have a 1998 v6 which started to do the same thing. I was told by the transmission shop this is not an uncommon problem with Ford products. We replaced sensors, servo, solenoid, etc., problem kept coming back. Last time it was a valve body repair. It's still doing it. Next step is a new tranny - more likely I'm gonna trade it in for a new Honda. Good luck!