I'm working on a Crown SP 3500 ac drive truck. It intermittetly slows down to a crawl once or twice a week for maybe 10 seconds then goes back to normal speed. Platform is all the way lowered when this happens and moving the throttle handle doesn't effect speed. There are no height switches on this truck ,it has a height encoder. No codes are registered when the truck is moving slow. Any one have any ideas? Thanks.