I have a 2000 SSEi with 74000 miles on it . Only mod is 3.4 pulley. Coming home from work it shifted fine. Parked for a few minutes then left to go to dinner. Transmission shifted hard. Every shift was a jerk. The car sat for about a half hour .Coming home it shifted fine again. What is about to happen? I checked the fluid and seem to be where it is suppose to be.

What color is the fluid?

From what I could see it was red. It did not smell burnt.

It could be the pressure control solenoid starting to get sticky. Has the fluid and filter ever been changed? If not, start there. You may want to put in some Seafoam TransTune first, run that for a short while, then do the fluid and filter change.

The pressure control solenoid is a known common problem. I found an explanation but was unable to find it in a search. Anyway the valve apparently starts to leak and there seems to be a sensor in the transmission that gets an erroneous reading due to the leak. This in turn causes the PCM to command too high a pressure and thus the hard shift. I am surmising that this erroneous reading is also the reason it will set a code. Plus, because it is commanded from the PCM due to the erroneous reading, the next time you start the car it gets reset and all is well for a while.

If someone knows where the original explanation is, please link it here.

It'* been discussed several times, but here is a link to the TSB on it.
A/T - 4T65E Poor Performance/Harsh Shifts/DTC'* Set

I did as suggested. The seafoam trans tune seem to do the job. I will drive for a few more miles and then change the fluid and filter.