Here are a couple of things to check. First make sure your throttle linkage is not binding and the carb is closing all the way. Also put a throttle return spring or band on it to ensure it also closes in the event of servo failure. Second item, Check the clutch and make sure it is built correctly. Ensure the bearings are good, the clutch bell should have about a 1mm of play back and forth and should spin freely when you spin it by hand if the engine is removed from the car. If it does not, it is not shimmed correctly or you have the shoes assembled with a problem. If the shoes are engaging the same thing will happen that you are referencing.