Car starts to miss and is slugish on accelaration , also # 6 cylinder is fouling !

I have a 1977 datzun 280z that have owned since 1978 , 2nd owner, The car has always been reliable , it became hard to pass smog here in california , so i had the engine rebuilt and it smogged fine again , since about 2000 i have not driven the car that much , may be about 10-20 miles a month , i know its insane , suddenly it does not pass smog again ! it always starts and is fine below 3000 rpm ! does this sound like sticking valves or what ? Any ideas would be wonderful ,thank you!