It could be a vacumn leak somewhere, from one of the lines or the intake manifold gasket. Is it an intermittent rise in tickover or is it steady? If its steady it could be as simple as adjusting the tickover screw. Some peope like to raise the tickover or tune up the radio to cover an idleing problem. Last Summer, my girlfriends dad made it home in record time when a washer jammed the carby in his Camira. He was a bit white in the face when he came in though.