83 Evinrude outboard question

I am new to boating and this group. I spent a couple of hours searching
this groups archives for information on my problem and found a couple
of "kinda like" matches but would appeciate feedback to help me decide
next step. (kinda matched lower unit slipping and fuel restriction
posts in the archives).
Bought a 83 Funtoon pontoon boat (28 foot) which came with an 83
Evinrude (60HP) late this spring. Ran just fine til this month.
Motor has developed a quirk that I would appreciate help with.
Motor starts fine, idles fine and runs fine at low to half throttle in
forward and reverse.
But,, when I push the throttle to forward full throttle the boat surges
and then falls back to low speed over and over.
The cycle time on the surge and fallback is a couple of seconds and
varies a little in length.
Boat will keep this up until throttle is reduced to half speed ot less.
It gets us around but we are starting to get whip lash (smile). If I
throttle back it purrs right along all day long. I get this surging
effect no matter how slowly or quickly I advance the throttle.
The motor tested OK on compression and had a new water pump and rebuilt
lower unit installed before I bought it per the seller and local
mechanic. Also had new fuel lines and primer bulb and battery.
I do not know motor hours in use or how it was stored.
I would really appreciate information and leads on calming this old
Evinrude down.