EGT / Tach Fluctuation

I am stumped as to what could be causing thing and am hoping someone
here can help me out.

A couple of flights ago I noticed that my EGT gauge started
fluctuating. Not fast but up and down a couple needle widths every
second or two. Then on the next flight I noticed that the tach has
started as well. Up and down 25 rpm or so.

I just had a cylinder overhauled about a hundred hours ago and I do
have have a much larger drop in the right mag that I need to clear to
get to normal on each flight.

My mechanic looked at my plugs last week and cleaned one he said that
had oil on it from the overhauled cylinder. I took it flying today to
see if that cleared the problem but it is still there.

Any ideas or thoughts?