My sisters car is acting really weird - when she leaves work in the morning about 6am she drives home at 60+ miles an hour it starts lugging like it's loosing power after about 15 miles or so. If she stops the car on the side of the road and lets it sit running for a few minutes then she can start up again and got another 15 miles before it starts lugging again and won't hold the speed. Has anybody seen this -her car is carborated, and it only does it during the winter. Any ideas????? Please help!!!