First, I think its time for a rebuild, partial or more full.  I live in Southeast Wisconsin just north of the Illinois border south of Lake Geneva WI, West of Kenosha WI, and East of Beloit WI.

Anyone recommend a shop?

Also, and ideas of what I have happening?

Compression drops as speed increases, or is this just the measurement you've taken over time?

Sounds like valves to me... especially the dropping one cylinder above idle, and more as throttle increases.

What's probably happening, is one intake valve is either lazy (weak spring, gunky guide) or the clearance is too tight, so it's leaking... and once you get it running faster, it's lazy, so it leaks more, which screws up manifold vacuum, causing others to run lean.

Pop the valve cover off and give the rockers a basic finger-wobble on all valves that're down, then push down on every valve that's up.  Turn the engine a full rotation and repeat for the others.  Notice any difference in any?  clearances and spring reactions should all feel about the same.  Valves should snap shut with authority.

Now start the engine, let it idle, and watch each rocker.  Bring it up off idle so it's dropping one.  Take a wooden hammer handle, and place it atop the rocker over each pushrod, one at a time, and lean on it a little.  work your way down.  Does the one missing cylinder pick up?  If so, you found your culprit.  Shut it down, open up clearance a bit, and try again.

Because thats the proceedure for a compression test.....On 2cycle Detroits,you put the tester in place of an injector and ran the engine but thats apples and oranges. Gas engines are not tested running.

>>How is compression tested at full throttle with all spark plugs removed?  This I need to see.<<

Stop engine.  Connect appropriate battery charger to battery. Remove all spark plugs.  Advance throttle and choke to fully open position.  Install compression tester to 1# spark plug, engage starter, observe and record gauge pressure.  Move tester to #2, #3, and #4, repeating for each.

When done, disconnect charger, replace all plugs.  Oh, actually... look at each plug... note the color of each plug.  See any that're substantially different?  That's usually an excellent clue towards finding problem areas.  Look at plug gap too, and make sure plugs are all the same type.

After you install all the plugs, start and run the engine.  Spray a little WD-40 or similar on the base of the plug insulator, right where ceramic meets the metal.  If you see bubbles at the crimp, replace the plugs with something other than Champion... don't ask me why

Another thing you might wanna look into...

Is this a distributor, or magneto ignition?

If distributor, take the cap off, grab the rotor, and wiggle it left right and foreward-back (not up down or twist).  Does it move a little?

It's not unusual for the distributor shaft bushing under the advance mechanism to wear over time.  The previous oil may have been thick enough to keep it from rattling around, but the new oil may have granted it new freedom, and now it's wandering around, and causing the ignition points to not be reliably lifted by the point cam. Same is true for the distributor drive gear... if it's well-worn, it may be that it's rattling against the gears and you're getting inconsistent timing as a result.  On some engines, the oil pump is in the pan directly below the distributor, and the distributor gear's loading is changed by different grade of oil, and of course, multi-vis oils changing with running temperature...

  

Its a magneto.  I still need to get the timing correct.  This engine was rebuilt at one time and the internal timing gear that connects to the magneto is not in its correct position.  I had to replace the magneto this year and am still fiddling with it to get it timed perfectly.  It starts right up, and hasn't had this new missing problem at higher RPM until recently, and it happened suddenly. I can be running for quite some time and all of a sudden at higher RPM and under load it will start to miss and then stall.  I learned that if I back the throttle down it will keep running, most of the time, or push the clutch in and take it back to idle before starting work again. 

Thanks Tucker.  That's good information and advice