I am experiencing the same problem as the others here.Transmission shifts fine until it warms up to operating temperature and then starts to shift hard through the gears.It doesn't always occur so i am assuming that my pressure control solenoid is starting to go. Ive seen a few pictures of what this part looks like, Its made out of plastic with what looks like an aluminum assembly but not sure.....it looks like its made mostly of plastic so no wonder they fail often even if the transmission is maintained with fluid changes and filter. I see now why GM is losing customers because of cheap parts most likely made in China to save on costs. I will never buy another GM product again because of my experiences on this vehicle!

For anyone who comes here looking for help with the Monte Carlo or similar GM vehicle which "Bangs" "Clunks" or "Slams" into gears ( you may have noticed your transmission shifts ok for the first few minutes after you start it) :

This is caused by the failure of the PSC or Pressure Control Solenoid. This solenoid resides inside the transmission and regulates "Line Pressure". When this solenoid goes bad, the computer notices the transmission is not shifting right and defaults to what is known as "Max Adapt / Long Shift" mode in which the line pressure is maxed out. This is what causes the harsh shifts and this is why the car will drive smoothly for a few miles after you start it and then shift harshly.

Our car did this on the initial drive home (bought this car with 103K) and I followed up on it right away. Be AWARE -- any shop that does not have a GM SPECIFIC code scanner will NOT SEE THIS COMPUTER CODE (P1811) and you will NOT get the proper diagnosis. This happened initially to me because I bought this car from a Ford dealer - I knew this ahead of time and explained it to the service manager and the car was sent to a GM dealer where it was accurately diagnosed. This was the only way this car would have ever been fixed right!!!!!

This repair requires removal of an access cover which is not easy to get at. The charge for this job is around $600, with most of this cost due to the labor involved. After this job was done, the car has been great! No problems at all in the last 20K, and we did get a local garage to do a Chemical Flush of the transmission. Again, no problems, but if your trans fluid is dark, smells burnt, or has gritty bits in it, it may be too far gone to fix like this.

Good Luck everybody! And email me at jeffsherry@charter.net if you need any more help. We love Monte Carlos, from 1970 to today(I am partial to the RWD cars).

Had Trans flush twice. Car is great, except for the trans clunking. First started in third gear, now in first and second. Worse when engine is warmed up. Who would think that a 6 year old CHEVROLET would have trans problems! I cannot find any GM Recall on this problem..

Had a mechanic run codes on it, I have the print out and will take it to the Chevy dealer where I bought the car. The car is maintained faithfully with oil changes, filters, plugs and wires. Also had to replace the oxygen sensor - at a cost of $278. This, all of course happened after my extended warranty ended. Currently have 107,000 miles. Remember when Chevys were KING?