We have a 99 chevy suburban (1500), which has been not starting intermittamtly for the past 8 months. during that time we have had a complete tune up and replaced 3 fuel pumps. The first 2 were bosch pumps, and recently we spent big bucks for for a GM pump. The car never stops when running, but after sitting for a while. It also sometimes will start after sitting a few days. We are afraid to drive it. Any ideas on what might be wrong?