Have you checked your transmission fluid level?
Checking the transmission fluid level is non-trivial.
1/ Fluid temperature must be between 30-50 degrees C / 86-122 degrees F. (I guess fully warmed up)
2/ Engine running, hold down brake pedal firmly and move shift lever through all ranges, leaving it in each range for a few seconds; return to park.
3/ Raise your XLR on a hoist, engine running, car exactly level.
4/ Remove transmission level hole plug (caution, hot!). (There seem to be two plugs, one at the front and one at the rear of the transmission assembly, above the sump level in the left hand (driver's) side of the transmission casing (this is from a 2004 XLR)).
There is an extended warning about doing this, basically the engine must stay running or hot oil will pour out of the level filler.
"Caution, the engine must be running when the transmission fill plug is removed, or excessive fluid loss will occur. Transmission fluid may be hot. Since the actual fluid level is unknown, stand clear when removing the fill plug. Have a container ready to capture any lost fluid. Do not turn the engine off with the fill plug removed, as you can be injured by the hot transmission fluid being expelled out of the oil fill opening."
5/ No mention is made as to how you actually check the level, I guess fluid should just drip out. I would be very reluctant to stick my finger into a moving transmission to feel for the fluid level and anyway, it will be hot!
6/ If the level is low, add Dexron III in 0.5l / 0.5qt quantities until it drips out. (Suggest you read this site re the proper fluid, there have been a number of tech bulletins on the correct additives etc.) (Does anyone know, is Dexron IV + additives or Dexron LS recommended now?)
7/ If you add more than 1.5l, check for a leak (it says). (My view would be that even 1.5l is a lot).
Perhaps others can add their views?
Harsh shifts can be caused by a number of factors, the best advice is to get those codes scanned, see what shows up. Output speed sensor, input speed sensor or throttle position sensor are candidates.
My preference would be to scan for the fault codes before I tried to measure the fluid level.