Occasional Poor Diesel Idling

My truck is a 2005 with the Duramax Diesel & 6sp manual. Normally, it idles about 750rpm. Occasionally, the idle speed will drop to 675 rpm or so and the engine will shake terribly. Lightly depressing the
accelerator will return things to normal. Fuel economy is great for this size truck. I get about 24mpg cruzing empty on the interstate at 57mph. The other end of the range is about 11 mpg towing a 14000 lb 5th wheel through the mountains.
I have checked for codes when this occurs and none have been found so far. I keep checking everytime it happens. As expected, the truck always idles normally when I stop by the dealer.
The only idea I have is possible water in the fuel. I am currently using a fuel conditioner recommended by the dealer. Maybe this will work.
Does anyone have any suggestions? Dave
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Simple, easy and (relatively) cheap problem to fix - change you fuel filter. Several bad batches of fuel from the same station could have picked up enough junk (dirt, sediment, algae, etc.) to easily clog your stock filter, and poor idle is the first symptom. If you had any appreciable amount of water in your filter/seperator unit it would turn on the "Water in Fuel" light on your dash.
Although a good fuel conditioner can be beneficial, don't go overboard with it. My personal experience is that I have the best results when I use it on the lean side.
I also recommend using a second (auxillary) fuel filter with a 2 micron rating, and insisting on the newer model stock filters with a 2 micron rating (the older OEM filters had a 10 micron rating). There's an easy to mount Racor aux fuel filter kit available at www.lubricationspecialist.com.
Cheers - Jonathan
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