" not going into gear on startup " by that do you mean it takes a second before you feel it engage in gear when you shift into drive or reverse ? .. sometimes just a pint or so overfill mite fix that .. if it still shifts firm at 1-2 and 2-3 shift points it obviously does`nt have serious problems .. not likely that an engine overheat will damage the trans .. if you have slippage the fluid will scoarch pretty quickly ..