Help; Rough running Continental O-300D

I hate to keep bugging this group with my bizarre Cessna questions but it
repeatedly proves to be a good source of information.
History; Since I got my '61 C-172 with an O-300D several years ago it always
ran smooth but had high oil consumption. Before the second annual several of
the cylinders started to show bad compression. Rings were all fouled up and
frozen. Replaced rings and it continued to run great with no oil use for about
3 to 4 hours then went back to using oil (been through 2 annuals now,
compression still good). 8 months later right mag goes bad (cutting in and out
on several cylinders). I replaced the mag and do a timing check. Find both mags
were over advanced about 12° (seems since it ran smooth my AI never bothered to
check timing. I should have done it myself but I thought that was what I was
paying for.) This, of course, was why it used oil. Reset timing, and another
compression check revealed 2 nearly dead cylinders. Broken rings on one
cylinder, frozen on another. Other 4 cylinders were 80/70 or better. Installed
new rings on those 2 cylinders and power great with virtually no oil use.
Strange problem; Idle at 500 RPM is smooth as silk and so is operation over
1700 RPM. Tons of power and 100° cooler head temps to boot. Operation between
1000 RPM and 1700 RPM seems very rough (never did this before).
I doubt it's an air leak since it idles so smooth. Idle mixture screw is out
less than 1 and a half turns so I don't think it's compensating for something
by over richening. Mag check at any speed shows even RPM loss not over 100 RPM
at worst. I could swear it feels like a cross fire (it's that bad) but why
wouldn't it do it at higher power settings? I also find that hard to imagine
with shielded wires (meaning they would short to ground, right?) I thought the
tertiary idle delivery circuit (MA-3SPA carburetor) may have picked up debris
but I had the carb apart to the last nut and bolt and everything and all
passages are clean as a whistle. Seems like this never happened until the
timing was backed to the standard 28° left and 26° right. Anyone seen this? Any
ideas? What is staring me right in the face that I missed?

Jim "boy I like my Lycoming" F