Eratic engine idle when warm

["dmcman82" <srubano@xxxx>]

Hello all,
I am experiencing a problem with my D that has me puzzled. I started 
it up for the first time in 5 months and it started with no problem 
at all. as the engine sat idling for a few minutes until it reached 
operating temperature it ran like a champ. As soon as it gets to 
normal operating temperature the idle starts to fluctuate between 
7000 and 10000 RPMS. As it fluctuates I can hear the Frequency valve 
slowing down almost stopping at times (this is where the engine 
starts to drop in RPM) then the Frequency valve will jump "alive" 
again returning to normal (where the engine will idle normally). 
This is a continuous cycle of the engine going up and down in RPM's 
and every time I hear the frequency valve dying out then returning 
to normal again.

Now my questions: 
1. Could there be something wrong in my Lambda circuit causing this 
such as a bad O2 sensor?

2. Could my frequency valve be bad?

Either part is expensive so I don't want to "swap it out to see if 
it fixes it" and later find out it wasn't it at all. I am assuming 
that if the Frequency valve would be bad it would just not work 
which leads me to believe that there may be a problem with my O2 
sensor? Does this sound logical? 

My CPR valve works like a charm. When it's cold and I start the 
engine it will start off at like 1-1.5 Bar and slowly work it's way 
up to 3-3.5 bar as the engine warms up. My fuel pump, Accum and fuel 
filter are all new as are the fuel lines. I know it can't be my Fuel 
distrib (or could it?) since the engine runs fine until it warms up 
to operating temperature. 

Any suggestions? Thanks in advance.

Steve