Hard Shifting

My 2000 Bonneville SE (automatic) seems to shift abnormally hard. I have 66,000 miles on it and I have noticed recently that the car shifts hard as I increase in speed from 0 to 50. The first shift isn't so bad (around 15 to 20 mph) the second shift is a little harder (around 25 to 30 mph) and the third shift at 45 to 50 mph is the worst, almost like the car is trying to catch up to the engine. The problem becomes more prevalent after the car has warmed up. I would greatly appreciate and input to why this is happening. Thanks

Sounds like it's either low on fluid or perhaps the Pressure Control Solenoid inside the trans has gone. Is the Check Engine Light on?

I am having some of the same problems on my 2000 Bonniville. After the car is good and warmed up and I am driving along at any where between 35 and 50 miles Per hour the car jerks, I carefully noticed that the RPM's don't change when this jerking action is present. And pushing on the throttle doesn't help. After four or five jerks it stops and then it will do it again a little later.
I only have 50,000 miles on this car, the third gear went of out it and was handled by warranty several years back. The engine check light has never come on, but this jerking motion is getting worse and happening more often

Can this also be the pressure control solenoid?
If so, can I replace the PSC myself?

Thanks for your help