7.4 MPI (310 hp)
NO fault codes stored on the ECU
but when running on idle speed
diacom screen (right side page) flashes/varies on the line "RPM DROOP" ON & OFF all the time
when engine running faster (+1000 rpm) "RPM DROOP" stays on OFF
why @ idle ?
What does rpm droop means?
How to solve this issue?

Your idle is falling below preset minimum. Not knowing a serial number or year it is probably 600, so its falling below

Look at the IAC and what are the counts and is it steady?

I ordered new IAC and let you know
thx for the help/info

Have you checked base timing against what your repair manual says it should be?