shuttering and pulsing at cruise

Dynamic Prop Balance ?

Prop & Blade Security ?

Prop Pitch ?

Engine Mounts Condition?

Prop pitch set with digial meter.

Prop dynamically balanced.

Mounts look fine.

After 15 years of flying I dont know that I have ever been aware of what an out of balance carb sync feels like as it has always been extremely smooth.  What does the out of balance feel like at cruise?

Six pulses per second is 360 rpm, not sure what would turn that slowly at cruise power (what engine rpm is your cruise power?). Definitely not the prop. I’ve flown with a slight carb synch imbalance at cruise and never experienced a noticeable tugging or unusual noise- have you mechanically and pneumatically synched and still experience the problem?

 "Mounts look fine."

Looks can be deceiving.

Rubber can soften with time & movement/use.

Have you tried a vigorous side to side & front to back movement test on the engine?

When were the mounts last replaced????

Have another look at the plug wires and caps. Could be an intermittent short at high rpm. Had a similar experience when a plug wire was grounding on the intake manifold.