Gotta assume you have a auto tranny & a V-6. Gotta give us all the info, dude.
The trans will shift HARD if you overheat the fluid. If you were low, that is likely what happened. Perform a couple of drain & fill sessons over the next few weekends to clean up the fluid & add a tranny cooler to prevent future problems. Dad the old school mechanic should be able to help you with this relatively easy task. Get a stacked plate cooler for best cooling. The tube & fin designs are far less efficient, but they are much cheaper and seem to do the job in most cases. The tube & fin is also pretty tough if you encounter dirt/gravel roads from time to time.
The auto trans also has a condition known as a 'bang shift'. It has to do with a combination of the Idle Air Control (IAC) valve not closing properly and possibly a weak signal from the Turbine Speed Sensor (TSS). Usually easy & cheap to resolve. All it took for my Mystique was the Throttle Hang (TH) fix. Costs pennies & relatively easy. If the condition persists after the TH fix, then get a new TSS and replace it. Takes 5 minutes if you can squeeze past the driver wheel, a little longer but easier if you have to take the wheel off. Cost about $20.
Good luck, and welcome back to the boards!