arrived on site to check top Middleby Marshall conveyor oven. customer doesn't have oven in operation. brought oven into operation and found during conveyor running, conveyor would slow and stop by immediately start again. proceeded and checked optical sensor okay on motor. checked speed controller and found extremely hot to touch. customer said that exhaust fan over ovens is currently out and that they are awaiting technician to repair that. replaced speed controller and display. brought oven into operation. set time on display to 6:21 and allowed oven to heat and run. conveyor running smoother. but, customer needs to get exhaust fan repaired. oven in service.