Sounds like an issue that will take a good shop to diagnose for you, but perhaps first things first.
Make sure the trans fluid is at the correct operating level with the transmission hot. Check for any signs that it is not the correct color. Nissan J will be sort of clearish, and aftermarket, usually red, but you probably know that. Any signs of coolant in the trans fluid (looks sort of like a milkshake) or dark, burnt fluid and/or odor wouldn't be good, of course. If the radiator fails internally, it can allow coolant to enter the trans fluid cooler circuit, so that's a worry until the trans fluid is checked out OK.
If you've already checked all that, then I think it's time to head into the shop, unfortunately.
Good luck.