My 89 limited 2-door has always shifted hard into overdrive. Lately, however, after about an hour of driving, it feels as if the transmission is slipping between about 40 and 55 mph. It doesn't do this when it's cold, only when it's been driven for about an hour or so. Can anyone tell me if she's a goner, or if maybe changing trans fluid and filter could fix the problem?

That is a good idea also lets u see if there's schrapnel in bottom of oil pan . If this is an electronic tranny could be sticky solenoid . Later bake .

changed fluid and filter and she's shifting ALOT smoother, but still when enging is good and warm (not overheating) seems to be missing/slipping when traveling at steady speed. Could be computer?. . . sensor?. . . also just changed plugs about 2 weeks ago. Thanks for all your help

try a bottle of lucas trans fix.

try a bottle of lucas trans fix.

X2 on the trans fix. How many miles are on the trans? Its possible that the clutches are wearing out.