My Mercedes sprinter wont go into gear?

I have a 2001 Mercedes Sprinter when I turn the key on I get a "F" in the gear selection light. This will not allow the ignition to start, however if I hot wire it I can get it to start, however I have to hold the gear stick in "A" while I drive it. when going slow however it sometimes still jumps out of gear and goes into "F" mode. I have to stop and wait while it takes time to go to "N" and then "A". Has anyone else had this problem. We took it to a diagnostic place but they couldn't pinpoint the problem