I loaded a yard of drain rock in my 2003 duramax/allison and parked the truck for 3 hours. Start truck up and there is no light under the P R N D indicators and the check engine light comes on. Truck goes into limp mode, one gear, can't go over 45 MPH. I drive it 10 miles pull over and shut it off and restart, no change still in limp mode. I drive it the rest of the way home, 22 miles and park at the Post Office and go in. Come out and restart the truck and the indicator light is now working and the truck is no longer in limp mode. Check codes and have a P0700 (2X's) and a P0708. Check manual and it says neutral indicator switch, check wires everything is OK. Clear codes and truck is running great. What is up? Any ideas?

Thanks
Patrick