The only thing you can do is to disassemble it and clean and relube the gearbox. If it is intermittent in the fact that it only goes up sometimes, it may be the relay, or the radio itself, as long as it goes down the cable is usually fine. The way you can tell if it is the cable, is to pull on the top of the mast when it is halfway up. If it pulls up more than a fraction of an inch, the cable is broken. It should push and/or pull about 1/4 of an inch. The movement is the armature moving to actuate the internal cutoff switch.
The antenna isn't usually intermittent, it either works or is does not, although it is possible that the brushes are worn, and not making good contact with the armature. The brush arms can be bent a little to increase the pressure on the armature. There is also a switch that the motor actuates when it reaches either end of the travel. If it is sticking, opening the case, cleaning, and lubing is the only cure.