. . some time the transmission slips when

You may be low on transmission fluid or the transmission is goig to fail.

If it keeps happining check your fluid and if its low every time u have a leak somewhere.

How many miles on this transmission?

Perhaps it is in severe pain, because you failed to identify it it by make and model!

Servo or band worn out, o.ring gone hard or clutch pack burnt out.

If the transmission is slipping, the FIRST thing to do is check the transmission fluid level and make sure it is full. If it is low, top ot off and see if this helps the slipping problem. Then find out why it was low (leaking) and get that repaired.

If the fluid level is not low and it is slipping, then it is probably time for a transmission overhaul.

Depending on the amount of damage done driving it low on fluid you may be able to salvage more use out of it. The first thing you gotta do is change the filter and gasket inside the transmission pan. Check the owners manual and be sure to replace it with the correct type and amount of automatic transmission fluid. It might even help to get your local mechanic or parts store to recommend the best transmission additive available. After and only after you've done all this. You wait until low traffic time and drive it in the immediate area until such time as you can tell that it's shifting and not slipping at all. If it has "any" slip it will only continue to get worse. Don't let anyone sell you some 6 buck a quart fluid for the change unless that's the only one the manufacturer recommends. Good luck. This repair, if you are mechanically inclined and have a suitable jack to safely get under the car to change the filter, shouldn't be bank breaking. You may have to go to your local mechanic. But even a hundred or so is better than the pocket emptying transmission overhaul.

you have worn clutch plates inside the direct drum for the foward gears. they last on average 100,000 miles give and take 20,000 miles

SOUND LIKE TRANSMISSION IS SLIPPING.CHECK THE TRANSMISSION FLUID MAKE SURE ITS NOT LOW.ALSO IF TRANSMISSION FLUID LOOKS DIRTY DARK AND SMELLS BAD REPLACE FLUID AND FILTER.IF FLUID IS GOOD YOU HAVE.TRANSMISSION OIL PUMP AND FORWARD CLUTCH PROBLEMS.

check the transmission fluid, it may be low, if not look and see if it looks black if it does you may need to change the transmission filter and fluid