Ok, my Cummins was running rough occasionally.  I take it to the
dealer last week, get a bit of a bs story, a "fuel system" cleaning,
and sent on my merry way.  Oh, BTW, it's got about 2,800 miles on it,
on '06 3500.

Today, I was sitting in the truck, in park, running the engine at
different rpms.  Idle seems to be set at about 800 rpms and it will
hold that steady.  If I push the accelerator pedal to get the rpms to
1,000, the rpms will go from 1,000 to just about 1,100, then back to
1,000.  The cycle repeats at about 5 to 7 second intervals.  Other
than at idle, there doesn't seem to be any rpm I've tried, from 900 to
2,000, that this doesn't happen.  It is virtually impossible to hold a
steady above idle rpm and the variance is anywhere from 50 to 100
rpms, at  5 to 7 second intervals.

I doubt if this is normal, but if it is, let me know.  Otherwise, I'd
sure appreciate any opinions on here about what might be happening
before I have to go back to the dealer.

Thanks in advance for any opinions.