Whats up with my transmission? When cruising along it just starts downshifting from OD back to third (back and forth). Will stay in OD with my foot on the go pedal but when I let up it downshifts again. Never did this before. This transmission is new (15,000 mi. ), with a DTT torque converter and Superior shift kit. What do I check for? Fluid level is correct, truck is a 1993. Thanks.