Transmission repair can be quite expensive. Give this Transmission Troubleshooting guide a read.
You might just find a quick answer and solution to your transmission problems.
Over Filling Transmission Fluid
Overfilling transmission fluid can cause the transmission fluid to foam up, the term for this is called cavitation. Transmission fluid cavitation happens when the transmission fluid level is too high, this causes the fluid pressure to change creating cavitation bubbles. Symptoms of a transmission cavitating are erratic shifting, internal transmission damage and oil starvation.
Low Transmission Fluid
When a transmission has low transmission fluid, this cause the transmission oil pump to run dry. Once that happens, the torque converter that creates the pressure runs dry and there is no more hydraulic pressure to the solenoids which will cause missed shifting and also erratic shifting. This causes the friction plates and clutch bands to prematurely wear at a very fast rate. You'll know this by dropping the transmission pan and feeling for any grit at the bottom. You'll feel a bunch of fibers fibrous materials. This means that you're friction plates have been destroyed. One of the most common signs with low transmission fluid is, your transmission will make a whining noise in all gears including Park and neutral. Other signs of low transmission fluid can be slipping when you push the throttle down, or while you're holding a steady RPM your transmission shifts up and down erratically on it's own. With some automatic transmissions you are able to check the transmission fluid with a transmission dipstick. Other automatic transmissions do not have dipsticks, they have a fill hole level. You fill the transmission until it begins to pour out of the hole.
Dirty Transmission Fluid
There can be a few things wrong with transmission fluid. The transmission fluid can be contaminated with metal flakes, this will cause the internals of the transmission to wear l prematurely. If the transmission fluid has a burnt smell to it or is dark black, this it can prevent the transmission from shifting properly. Now this happens because transmission hydraulic system can't build up enough hydraulic pressure to shifting the gears.
Doing a proper automatic transmission flush along with a new filter and new transmission fluid can have the transmission shifting like new again.