I have a 1985 Chevy Silverado that I just bought and has a TH350 transmission instead of the TH400 that is suppose to be in it. Does anyone possibly know why my TH350 transmission keeps downshifting from drive to 2nd? The fluid is good, installed a new filter and gasket, replaced the vacuum modulator and has good vacuum. All the linkages are good, no binding. HELP!!!!