After the motor runs for about 10 minutes, when.

After the motor runs for about 10 minutes, when at idle(not in park) the car starts to loose power intermittenly. The tackometer jerks up and down. Advance auto has put meter on it and it shows the fuel is running rich or and they feel airs air is getting into fuel line. I have replaced spark plugs with original issue, also air intake valve, also air flow rubber tubes and gas cap. Advanced auto man says I need to replace gaskets in intake manifold. Do you have any suggestions? Note the car runs great even when Ii accelerate at a high speed!