Auto stick pops out of gear

I have a 68 bug auto stick I just picked up. great shape but it's been sitting for years. I did an over all tune up, new fluids, drained and flushed gas, valve adjust, new brakes etc. So she's up up and running strong. Reverse seems to work fine as does first and second. However third seems to "pop out" of gear at random times. Not necessarily while under load or hard exceleration. Sometimes while going up an average hill sometimes while just driving along. Sometimes it seems to not go into third immediately. I have worked on a ton of vw's (I have a 78 westy and a 70 beetle currently) but the auto stick is a new challenge for me. Any ideas as to the problem? Bad synchros? Does the auto stick have synchros?  ATF is new and filled within the proper range as is the engine oil. Thanks!