you may want to check to make sure you have good air flow across your radiator when the fan is running.make sure the fan is pulling air from the front of the truck and not pushing air out.as far as the mud! it can be very hard to get rid of and you may not beable to get rid of it short of having your block boiled out. I would try and have flushed professionally by a radiator shop then add a can of rust inhibitor made for cooling system. hope this helps (keep me posted)