virginia cambio

My 4200 CC I have to wait up to 30 min at times for it to start. Mind you this happens when I am leaving the grocery store, mall etc... It has gotten so bad that the motor will cut off while ideling or at low rpm's (like going over a speed bump). I have taken it to numerous mechanics (dealership and independent specialist) and they have not been able to disgnose the problem. I put in a new F1 pump relay and no change. It's generating no codes nor will it replicate at the dealers... I'm lost. Last night I left and drove a mile down the street to pick someone up. While waiting maybe 5/6 mins the car went into neutral and cut off. Wouldn't restart for 20 mins. After priming it 5/6 times it started on the next start. I hated to prime.It has been to the dealer two times. 2 mos and 3 weeks. They can't get it to duplicate. It conked out last night after a ten min drive across town and as I came to a stop it stalled out. I'm soooo frustrated at this point. Happens hot or cold.