Dtc P1870

For the past several years I've been having this code come up which is related to the transmission. What happens is after I've been driving for an hour or so on the highway and I come to stop light, as soon as I take off my car shifts extremely hard through the gears, mainly 1st gear. If I pull over and turn my car off for a few minutes, then start it, the problem goes away. This typically only happens in the heat of the summer and after a long drive. I do have a B&M transmission cooler installed which I thought fixed the problem for a while, but it didn't. I took my car to a dealership a few years ago and he said I needed a new transmission, I just laughed and left. Then I read somewhere the valve body may be sticking. Anyone know what the problem may be? I'm hoping for a cheap fix since my car is getting pretty old and I don't want to spend a lot to get it fixed.

Thanks.

I don't know the cause of the problem by here's the code write-up from Shoebox's site:

P1870

Transmission Component Slipping

*DTC P1870 will be stored in PCM history
*Inhibit TCC engagement
*Inhibit 4th gear if in hot mode
*Freeze 1-2 shift adapts from being updated
*Maximum line pressure

I had the exact same condition on my V6 Camaro for over a year before doing something about it. The problem is due to excessive wear on a valve in the tranny valve body. I took it to a tranny shop and it cost about $300 to fix it. They had a special reamer and oversize valve to replace the original. The other option is to install a Transgo shift kit. I believe it has a new valve that will also solve the problem.