Well, I got my Monte about a year ago and a little while after both the catalytic converter and resonator went out. My dad and I replaced both of them, but I've noticed when the car gets hot it shifts really hard. It gets really bad when its been driven for a long period of time on a summer day. (over the winter it was almost non existent) anybody have any ideas what is wrong exactly?

 

Our cars are notorious for tranny issues, they rely on line pressure to switch the gears. If you haven't changed the fluid now would be a good time, but do not get a full blown fluid change just the tranny oil and filter as if you get it full blown with the pressure you will loosen up the crud that is built up in the tranny and that could cause even more problems.

The issue you are having is related to the pressure control solenoid/shift solenoid, they essentially stick and then the pressure in the lines builds up and then they finally pop into gear causing the hard shift. They can be replaced, not certain on the cost of this job but here are two links to the replacements that you have available

4t65e Pressure Control Solenoid - Transmission & Drivetrain

Shift Solenoid - Transmission & Drivetrain

there are links on youtube on how to do this yourself, if you are handy with a wrench, you can do it in your driveway, but I postponed the issue and eventually had to pick up a new tranny, well used tranny but new to me lol, but I also got over 100 000 kms before it finally let go after the issue started and almost 300 000 kms on the car so I can't really complain. You can talk to a garage and see how much they will charge, but remember if you buy the parts online and supply you will save a bit of money as most garages charge a bit of a mark up on the parts they put in