Garage Cabinets, Garage Storage, and Garage Organization

I recently bought a 1993 2.3L mustang. After driving it continuously for at least an hour, the automatic gear system starts behaving in a funny way. Whenever the gears are changing a loud humming/grinding/clanging noise is heard from the gear box and the overdrive light on the dashboard begins to blink on and off repeatedly. One is forced then to drive slowly in gear 1 or 2. This happens especially if one is going uphill. But once you park the car and it cools down like overnight, then the gears work fine till it warms up again. I've checked my fuel filter, changed the hydraulic fluid, but the problem still persists and it's also baffled my mechanic. He's pulled out the gear box, replaced a few parts here and there, but still the problem is there. Please help!!