also, if your relays (that cut out portions of the resistor, so to increase speed of motor) are getting a bit old, they may click, but not make good contact internally and so not bypass the sections of the resistor they were intended to bypass.

There could be a few reasons, but since they are ressistors they do normally get pretty hot.

First of all, if the battery is undersized, undercharged or worn out that will make the ressistors noticibly hotter than normal. as a battery's voltage decreases the amount of amperage needed to do the same amount of work increases, the incresed amps result in more heat which leads to more resistance, which in turn can increase the amperage drawn from the battery, with a battery in poor condition this viscious cycle doesn't take long to begin.

Second, check the condition of the power cable through out the machine, ressistor controlled PEs haven't been made in a while, so I wouldn't be surprized to find a cable in poor condition due to old age; I frequently change the power cables from the ressistors to the contactor pannel because the cables have become old and brittle (not just brittle isulation but the conductor itself).

Lastly check the condition of the drive unit and motor. make sure that the brakes aren't dragging, this is uncommon on PEs, but can happen. also check for debris like plastic wrap and pallet banding wound up into the drive axel, this is very common and can create a lot of drag on the gear box.
Good luck