We own a 2000 Chevy Venture (102,000 miles) for 5 years now that started a real hard transmission shift about 2 years ago. Took it to a transmission specialist and of course needed a total rebuild at 1800 +$. We started driving another car and basically parked the Venture. Recently I had to put the Venture back into full time service and started researching the trans shift issue and P01811 code. The symptoms were the same as many with these transmissions..hard shifting ...whining pump...sometimes just fine. It seemed logical that the computer was timing the shift and determining a fault followed by an action to max out pressure which in turn causes a hard shift. So just before a family vacation I decided to install a TransGo shift kit and replace the fluid with Dextron VI. After which I drove the car on Vacation totaling around 600 miles...And wow...so far it is great ! Shifting is a little faster, as expected, but no more clunk and clank. The transgo shift kit isnt the least expensive manufacturer but they seemed to have many praises. I spent around $100 for the total job..including Dextron VI fluid. I want to say thanks for the information and I will Keep you posted of the performance.