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I have a 2001 Monaco Diplomat Motorhome with a Cummins 330hp ISC. Sometimes, when I remove my foot from the gas pedal, I lose my boost. When this occurs, pressing the gas peddle shows no increase in boost and black smoke pours out the exhaust. There is a small increase of RPM on the tachometer when pressing the pedal, but no power. When I pull over to the side of the road, just prior to coming to a complete stop (boost around 2.5 and 2 or 3 miles per hour), the boost suddenly returns and the coach acts as if nothing ever happened. No warning lights come on during this process. When it occurs, I can reproduce the situation from 1-8 times by removing my foot and letting the gas pedal return to idle position. When it has occurred, I have turned off the exhaust brake and cruise control, but it does not resolve the problem. Sometimes I can drive 600 - 700 miles with no issues over a few days. Then it starts and will occur several times over 30-60 miles. I have had 3 mechanics look at it, #1 found no codes and said the turbo looked fine (test drove it 60 miles with no failure), #2 felt it was a bad relay so I replaced the relay but the problem continues (no issues for a couple of days then occurred again), #3 found a code-I believe they said it was a Near engine shut down and stated it was a fuel filter or injector pump. He serviced the coach, replacing all filters and added some fuel additive. Test drove up a steep 9 mile hill and had no issues or codes when he returned. Started home the next morning and lost boost 3 times in first 10 miles, each occurred when I removed my foot from the gas pedal in rush hour traffic. The remaining 350 miles home I had no issues. The loss of boost is occurring both shortly after starting and when the engine has been running for several hours. I never have the issue occur unless I'm slowing down the coach and as long as I don't remove my foot from the pedal, it runs perfect. I don't think it is the fuel pump as it has no problem going up hills or pulling my tow vehicle. Anyone have any suggestions as to what may be causing the problem.