An old automatic transmission often starts slipping between shifts. This can be due to worn clutch discs or bands. If you're good at other car repairs, why not tackle and automatic transmission repair? A rebuild kit will set you back only a fraction the cost of a new or rebuilt transmission.
You'll probably need a day to pull the transmission (with help of an assistant), a day to take it apart, a day to inspect all the parts, a day to put it back together and a day to put it back in your car (with assistance again). If you've done this before, the time will be much less.
You'll also make a big mess with spilled transmission fluid so protect your garage floor with lots of newspaper and have lots of sawdust to soak up spills. You'll need lots of working surfaces to spread out all the parts in the order you remove them. A couple of cheap folding tables should do the trick if you need to improvise working surfaces. Finally, buy or borrow any special tools your manual calls for. If you're creative, you can probably improvise or even fabricate the special tools. Above all, be patient and attentive to detail when rebuilding an automatic transmission, and follow a good manual step by step.
Don't cut any corners. Inspect all hard parts, like shafts and gears, and replace them if damaged or worn. You may have to go to a car dealer or transmission shop to buy these parts, or pick up a sacrificial transmission from the bone yard. Replace that old torque converter with a quality new or rebuilt converter.