When was the last time you changed the fluid? You did not state the miles but will assume there could be more than 150K. If you have not changed the fluid or filter/screen that would be the first thing to do. There should be a drain plug on the pan. It may be one that requires an allen/hex wrench. Drain the fluid;about 3 1/2 quarts should come out. Replace with new fluid, and maybe a conditioner like Lucas. Drive it for a while, maybe even 500-1000 miles or so. Change the fluid again, and see if this makes a difference. Alternatively you could drop the pan and clean the filter/screen or replace that, replace the fluid, drive it then change the fluid again as suggested above. Probably will cost about $30 plus the filter. If this does not work you may need a more expensive fix. I have a 91' 6cyl. It was having a little problem getting into reverse when it got hot. Did the fluid change etc. Works much better but not perfect. KNow its on borrowed time but not in the mode for rebuilding the tranny at 250K miles. Good luck.