Try adjusting idle slightly higher. Also, stability improves as engine warms up, so if issue is only when cold don't worry about it.
Check actual rpm with phototach. If below around 550, governor dips into something best described as starting mode, where rack slams way up to get rpms to recover.
There are also problems with governor that can do this, but I'm not expert on the inside of the box.
Has it always done this? Was any work done that precipitated it?
Not familiar with cat IPs but following makers instructions for basic idle set including any bumper adjustments is what I would do to start.
Peter, funny but we are working on same thing. Note I have same engines and same sync, mine surges exactly as described (lead Engine on mine is starboard which is the one that surges) Once the engine warms the surging stops, about 3 or 4 minutes. Rather annoying until it settles down.
The Glenn system maybe adding delay in the feedback response. Set idle and especially bumper or whatever catcalls it the call Glenn it might need a different spring
Peter, did you make any progress with your engine surging? I noticed this for the first time last night on our boat. Engines purr at 500rpm but between about 700-1000 the Port engine was surging just a bit. I haven't noticed this before.