Wonky Reverse in 84 245

I have a 1984 245 wagon (auto) with 183,000 miles. The transmission works great in forward gears; shifts smoothly, etc. But when hot (especially after stop/go driving), and ONLY when hot, it's hard to get it to reverse... I shift into reverse, and it won't go until I rev hard (and then it jumps into reverse, and I'm guessing that's not good for it). Is this likely to be something a simple fluid/filter change might fix, or is it likely some linkage trouble or other problem?