First check the oil level. On newer units there was a site glass with temprature indicator also. Oil should be at least half way up the glass.
Older units you had to remove the fill cover and check the level by looking into tank.
Second, When is the last time you changed oil or filter? If the oil is milky it has water and needs to be changed. You should also change the filter on a regular basis also. If it is plugged up it could restrict oil flow.
If you have the units with a speed handle, try opening it up a bit, go one or two numbers higher on the handle and see if that does it.
You also may have to increase the run pressure, there is a pressure adjustment on the power pack, you have to remove a cap near the flow control block and tighten the bypass screw with a flat blade or allen wrench.
hope this helps.