It could certainly be in the swing box. Sounds like your brake ain't releasing or the motor is weak though. There is a solenoid valve that the coil could be week, as it heats up doesn't release brake properly? Could be the piston seals on the brake worn and when the oil gets hot it doesn't hold it released. Could be losing electric current to the swing brake solenoid. There are O-rings on the cartridge of solenoid valve. I would check voltage at solenoid, pretty sure it's 24v. Then ohm solenoid, i would have to look that up tomorrow but if it has really high resistence then that could easily be it. Then remove cartridge assembly from swing motor, check O-rings good and inner spool moves freely. After that, drain some oil off back/underneath and see if there is metal in the swing gear box. If all that checks out good....Probally the motor or brake. Best to let someone in your area used to working on that kind of stuff look at that part though.
Hope this helps....check out pics below to see how it is set up. Top two are solenoid then solenoid cartridge assembly. Bottom two are motor. You can see where i circled where the brake valve is located. :drinkup