Partial power-failure, slight vibration and surge. Cleared itself on low power Ideas?

[ao.frog]

Hi folks.

During a local flight yesterday, after approx 10 mins on stabilized 50*F LOP cruise at 3000', (55%: 21 inch/2350 RPM) I suddenly noticed a slight vibration and felt slight surge in the plane. (like slight power addition, slight power reduction, 1 sec intervals.) No surge indication on MAP and RPM. They where stable at 21/2350

All four EGTs's where indicating increasing LOP: approx 80-120 LOP.
CHT's where climbing from the usual 330-350*F to 430-450.

I turned towards the homefield (10 mins away), moved the mix towards full rich and started a slight climb.

That stopped the LOP to go further towards lean, but the CHT's where still higher than normal. And the slight power-surges and vibration continued.

Changing fueltank had no effect.
Ignition where in "Both" the hole time.

When approaching the field and within gliding distance, I slowly reduced power and passing approx 15 inches, I felt the vibration and surge go away.

After landing, I did a runup-check with all normal indications.

Tomorrow, I'll pull the cowls and start checking things.
Any ideas to where to start looking?

Setup is a Lyc IO-360, dual P mags, 220 hrs TT.

Thanks.