Always same cylinder? Might give Bill a call
To enhance Jim's input, PCM uses that function to maintain smooth idle under varying load conditions, such as a\c cycling, shifting into\out of drive range, etc.
To test it, kill each injector, noting and comparing rpm drop of all cylinders - weak cylinder will drop least rpm
jd
'96 Dodge 3500HD cc 2wd drw............'89 GMC 3500 cc 4wd drw
5.9 12v #10TST 6sp SBC13-1.375.......6.5TD EFI maxEtorq v2.0 DSG
DODGE makes it CUMMINS shakes it.....4L80E 205 4.10 Dana60\70HD
6 in a row makes it go.......................Grandpa's big truck