It usually happens after the engines been running for awhile. I tried adjusting timing and it seemed to have worked for awhile and then started doing it again. any suggaetions would be greatly appreciated.  

Sounds like it could be carbon buildup or it could be as simple as poor batches of fuel.  

You could try setting your idle mixture screws a little richer. I find I can make my engine run on when I lean the idle too much. Doesn't seem to do it on a richer idle setting.