I have a 94 f150 with 5.0 V8, 5 speed manual, 4 wheel drive, no A/C. Recently, while sitting at idle, the idle speed will drop suddenly and then pick back up to normal. Once in a while it will drop so much the engine dies. It starts right up again. While driving with little or no accelerator, the truck will buck badly. Under more acceleration it runs strong and smooth. No check engine light. I love this old truck. Can anyone help?