Erratic engine shuddering - tecumseh vantage 35?

hey, i have just recently rebuilt my tecumseh vanatge 35 mower and having not run it in years i found the following surprising, and am not sure whether it should happen or not.
When the engine is generally running, but considerably more noticeable with low RPM, i noticed that it would frequently make erratic shudders, that were much larger than the normal vibrations. It seemed that the lower the RPM was, the more frequent, and erratic these shudders were. Are they normal for that type of engine?
Many Thanks