Dirty Filter
If the Shop-Vac filter is dirty, then air will not be allowed to circulate through the vacuum, thus causing overheating. Open up the Shop-Vac. Unscrew the filter and inspect it. The most effective way to clean filters is to use an air hose. Blow air all along filter ridges to remove dirt. If the filter shows visible signs of wear and tear, purchase a replacement filter. Also remove any other dirt obstructions that may be around the motor, outlet, or inside the hose.
Motor Bearings
The Shop-Vac motor bearings need lubrication to run smoothly. Without lubrication the bearings will seize and create overheating. Open up the engine and locate the bearings. Place a couple drops of oil on bearings and work it around a little. If the Shop-Vac persists overheating, you may have a faulty or worn-out motor.
Overuse
Many Shop-Vac aren't intended for continuous operation. Depending upon certain businesses, a Shop-Vac may be turned on the majority of the day. This will cause the vac to overheat and wear out sooner than expected. Sometimes Shop-Vacs are used with a hose reducer to multiply sucking power; if used like this for an extended period of time, it will cause overheating.
Shop-Vac Customer Service
Call Shop-Vac customer service and explain the problem to them. Many times you can get the problem diagnosed over the phone and a replacement part sent free of charge. Some of the Shop-Vacs have lifetime guarantees and many have limited warranties, so check your warranty and see if you're still covered.