i have a 95 mitsubishi diamante 3.0 auto trans. car had been setting for two years trans was bad so i replaced the trans road tested it and it will shift two times then the engine will rev up i let off the gas wait a few seconds give it gas and it will do ok. i replaced the filter when i put the trans in and the fluid level is good.
What was the transmission doing before that had you park it? Did you put in a new, rebuilt, or salvage yard unit?
Are you saying that it is just hanging in gear between 3rd & 4th? Or are you saying it actually comes out of third but then just behaves like its in neutral?
Your best bet will be to have someone get it on a scanner, watch what is going on during the shifts, and check for error codes.