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Unfortunately you don't say how many miles it's done, which would be a good indicator as to whether or not it's even worth the cost of changing the oil . If say we're talking 150 - 175K, then maybe an oil change and filter will save the box. If it's more than say 200K you'll be very lucky IMHO. If you only bought it 2 weeks ago it is unbelievable that the previous owner didn't know there was a problem, and as such sold you a dud. A few words need to be exchanged.

1. I am no expert on these auto boxes (or anything else on these over computerised beasts, the combination of electronic and mechanical 'challenges' complicate things but having searched and read many problems concerning the transmission I would suggest the following approach. Being as it gets worse when warm / hot I would suspect an oil change might resolve the problem. Change the fluid and filter as you state. Do not add any 3rd party product to the oil initially, see how it responds to the new oil first. You must use ATF4. Whilst draining the old fluid check with your fingers for 'grit', if the fluid is smooth then it's a good but not conclusive evidence of a sound box. As far as I understand it the computer needs to relearn it's cycling after an oil change - didn't seem so when I did mine but have read that a number of longish runs are required including kick downs, up-hill climbs etc. to optimise the gear change 'point'. Also this gives the box's valves to be 'washed' with the new fluid and may clear any partial blockages caused by old contaminates. I wish you the best of luck & hope it's nothing more than ancient oil causing the problem. Please update the thread with any results to help everyone else. Richard