So I recently bought this 96 honda accord with 300k miles and couple days ago it started to slip gears let it cool down then it started to work for sometime then did the samething now it just revs, reverse seems to work fine its just when its in drive it just revs any idea on what it could be. Also had p0501 show up on the scanner

 

First, check that the transmission fluid level to make sure you are not extremely low. Add fluid to get to full if needed.

The P0501 is for the vehicle speed sensor. A faulty speed sensor shouldn't cause the transmission to slip gears, but would cause the car to shift at strange points in my opinion. I'd probably replace with a used part as I don't think this is the root problem.

If the fluid level is ok, I'd do a drain refill on the transmission to get fresh fluid in there. You need 9qts of dextronII or eqivalent. You drain about 3 quarts to empty the pan as ~4 quarts remain in the rest of the transmission. Drive the car and hopefully get through all of the gears. I usually suggest driving for a day or two to mix the fluid. I'm not sure if your car will be able to do this, but try. Repeat 2 more times to exchange most of the fluid.

If the transmission clutch packs are worn, then a replacement/rebuild is required which will be expensive.