my friend has a 2004 3500 with a 555 engine with just over 465,000 miles motor is all factory except for water pump and upgraded lift pump (went to newer style)

Anyways truck runs good except at idle it will sometimes almost dye and then it will catch itself and idle up to around 1000 rpm then it will idle smoothly again for a while then it will aventually do it again. Any idea where to start? Truck runs excellent other then for this.

 

have your batteries load tested.... although the truck may start fine... batteries that are weak in amperage can cause this condition...

 

What PMC says makes sense, are the grid heaters cycling when this condition happens? If so, then I would agree.

400+K, That's great, but if all stock, the whole high pressure fuel system has to be tired....

 

Just an idea but my buddies 2nd gen did this when his AC pump was starting to die.

 

Just my .02 but a failing FCA can cause these issues as well. With over 400k it is something to look at if it hasn't been replaced....mine was faulty with under 100k but the dealership told me the perverbial " we can't find a problem with it" story.....so after SIX times of taking it to them I just fixed the ding dang thing myself out of pocket and all has been well since....I'm now at 276k.

 

Like Jhenson mentioned I would take a hard look at the FCA. Classic symptom of a sticking FCA.

MikeyB