My transmission

You don't really think we can diagnose it over a computer, do you? What does "acting up" mean? I can think of 50 different symptoms, each with a different remedy. Your best solution is to take it to a transmission specialty shop, explain what it's doing or not doing, and let them test drive it and do some testing so they can give you an accurate diagnosis.

Dandy. The first thing to check is the fluid level. If it is low, the pump will draw up air instead of fluid. Since air compresses, the clutch packs will never apply properly. Even though the car might move a little, those clutch plates are slipping and that will lead to them burning out and you'll need a total transmission rebuild. Check the fluid when the engine is warm, after you've shifted through "1", "2", "D", "N", and "R", then leave it in park. The engine must be running when you check the level. Some fluid will splash up high on the dipstick giving you a false reading, so wipe the stick off first, then reinsert it fully, then pull it out to check the level. If it's real low, add some through the same dipstick tube, then check for where it is leaking. If it is just a little low, that won't be enough to cause the problem you're having.

If the fluid is full, the filter could also be plugged. Similar problems will result although it is less likely the clutch plates will be overheated, at least not as quickly. The next step would be to visit a transmission specialty shop. From their experience, a test drive, and some hydraulic pressure tests, they can give you a better diagnosis and estimate for repair.