car won't go over 20 mph

Get it scanned and see if there's a Throttle Position Code/DTC. Most common problem in this car is the throttle Body. But a transmission problem could cause this as well. Have the car scanned and see what codes they get. You try disconnecting both battery terminals, touch them together for a minute and connect them back. But I would tell you to get the codes first because by doing the battery reset, all engine and transmission codes will reset and your mechanic won't know what the problem was until the problem happens again.