I have a 98 Suburban 2500 Diesel with 100K miles. I had it at the dealer for the 90k/100K ($500) service where they flushed the cooling system. While pulling a camper up a long incline in the Rockies, she started overheating and losing power. This has never happened before - the week prior to this trip, pulled the same trailer up an even longer incline with no problems.

When I complained, they took it back in and said the coolant reservoir needed replacing, charged me another 150 bucks - they also replaced the 2 thermostats. Took the trailer up an incline again and same result - losing power, black smoke, and overheating.

What could these guys have possibly done to cause problems that had never shown up before?