Dodge Caravan 98 (a604) tranny problem

My 98 Dodge Caravan 3.0 v6 4speed transmission developed a major fault. While we were out after it was sitting idling hot, when I pulled away I noticed a whine from the tranny. After a short while at a stop sign I pressed the gas and it did nothing for a second then jumped or bumped firmly into 1st gear. I immediately got into the parking lot of a nearby carspares store and it was acting pretty badly, by not immediiately engaging into drive from a standing position, also making the whining sound (possibly torq converter?) I bought some lucas transmission fix, 'is this good for the a604 transmission?' No one seems to know.
anyway got the vehicle back on my inlaws trailer. Now its cold again the tranny seems to behave just fine. Should I just do an oil change and new filter? And hope for the best. Im told that if it has the wrong tranny oil in there (dexron) it may cause it to malfunction. (Previous owners of the car last did oil change) So ill go ahead and put atf3 in there. Also will a tranny cooler do any good, and are they easy and not to expensive, is it worth the risk on this. Seeing as when its cool it seems to operate ok. Thanks.