heres my issue, at the ending of the day on the last load on a very muddy work site my 2000 mack vision cx613 really struggled to take off loaded from a stop . still continues to this day, changed air filter, had mechanic check clutch to see if its slipping ,he said it's good and not needing adjustment. the transmission shifts normally, not over heating, rear end temps are normal. truck just hasn't got the power to take off loaded and it's even sluggish empty, dont know if a spring came off the pressure plate or could it be a joint, turbo sensor, turbo, I really have no clue any thoughts would be appreciated . it does put black exhaust out when shifting, not a real lot but does for about 5 seconds per year shifted