I have a 90 Blazer with 130K. It has no mods done to it. When I take a left or right curve the trans downshifts and normally up shifts before the curve is over. If I am going around a long curve it will down and up shift multiple times. I have sea foamed the trans and changed the filter and put new fluid in and it is still doing it. It seemed to be better after I first did it and now it is slowly getting worse.