This is a 2000 Forester Auto. with 143,000. The problem started years ago on a long trip. The car shifted down and then revved up to about 4200 rpm and then shifted up and back to 2900 rpm and then started the cycle again and again. I tried manually shifting with no success. My best luck seemed to be to be very easy on the accelerator although that didn't solve the problem completely.
Ever since, it will run fine at first but when it has warmed up, say about 8 to 10 miles, it will start this again. It seems to have much less of a problem when it is cold outside--especially in the winter months. We rarely drive it now except in winter.
History: replaced fuel filter, knock sensor, both 02 sensors, alternator, coil pack, transmission filter and fluids--no difference.
Switched out TCU with another working one--no difference. Removed fuse for AWD--no difference.
MAP sensor cleaned.
It has a "torque" binding problem in forward and reverse when turning at very slow speeds. This is nothing new.
My new mechanic wants to put a used transmission in it. I asked him why because through all of this the transmission has not become any worse, mechanically.
I am at a loss. I need some help. Why would it do so much better in Winter and do well for the first 8 or 10 miles until it warms up in other months?
Thanks.