I've recently purchased a 1988 F150 5.0/5 speed manual. It has 133k miles and runs strong, the transmission was rebuilt around 10k ago. Everything runs well, but when the truck is idling, it sometimes fluctuates from 250 and back up to maybe around 1200 RPMs. Sometimes it just sits around 750 which seems normal to me. It doesn't always fluctuate, but it sometimes does. The truck has been kept in a garage and only driven about 2,000 miles in the last 2 years. It's had new plugs and wires install within that same time frame. Has anyone else experienced this fluctuation of RPMs at idle, what do you think it may be?