912is rpm drop

I am experiencing a 1500 rpm drop if I shut down lane B and the engine runs rough. if I shut down only Lane A, I get a few rpm rise as expected.

in recent flights I have been experiencing rising EGT's on cylinders 1 and 2 but 

I can hold them off at around 1600 degrees if I pull throttle back to 4700-4800 RPM. During this time the engine will feel less smooth.  All of sudden the engine will clear up and run perfect and the EGT's will drop like a rock to 1400 degrees. it may run this way for a few minutes or more then go back to climbing EGT's on 1 and 2. I don't get any lane faults or change of fuel pressure. 

any ideas where to look for the issue?

Thanks,

Marty