Is this a new re-build with a new cam lifters etc.? Or has the engine been in the car and run just fine last year etc.? If the engine has been in the car and ran fine last year, what changes were made from when it ran just fine until now? Did you change ignition setups? Did you change the distributor? Did you add gas to the tank? It could be bad gas. I would pull the carb off and empty all the gas out of it and use dry air and blow the carb out as best as you can. I am with Andy F that some debri is probably caught in the carb. Especially if the car runs at higher RPM's with no problem. The debri is probably clogging up the transition circuit from off-idle.