[BC] Harris FM 3.5K tube vs. neutralization

[Tim Portzline]

I performed a routine PA tube swap with a Harris FM 3.5K last night, only to find that the PA input tuning suddenly became very sensitive.  I'm trying to determine if I'm dealing with the neutralization or if I have a bad tube (4CX3500A).  How touchy is the neutralization in this box?
 
About 30 seconds after turning on the plate, the first thing I noticed was that the forward power suddenly decreased to about 20% and the PA grid current dropped to zero.  Adjusting the PA input would bring the power back up to about 90%, but any further adjustments to bring the power up to 100% - either to the input tuning or the plate tuning - would result in the power suddenly dropping back to 20%.  I got several IPA overloads as well.
 
As a side note, I noticed that the modulation monitor, which gets its feed from the TX in question, put out distorted audio while I was making adjustments.
 
I put the original tube back in to make sure that nothing in the unit failed in the process of juggling tubes.  It came right back to its prior condition without any problems.
 
What do you think?  A) bad replacement tube?  B) neutralization?  C) something else?
 
Thanks,
 
Tim