Dead Cylinder Below About 1500 rpm's

Back in the fall of 2012, I had a shake from idle to about 1500. Plugs and wires were fine. I did a compression check and it showed that no.6 had barely 90psi - while the others were from 120 to 130. Valve lash was okay, and the head was rebuilt just prior to me getting the car 8 years earlier. I'd only put on 6 thou during this time.
I then did a 'leakdown' test and there was lots of air flowing into crankcase. A couple others also leaked that way but not as much. There was no 'hissing' from any of the valves.
That pretty much indicated there was a 'ring-sealing' issue. I removed the head and then orientated the pistons so all were below the deck. If I poured some light solvent (such as Zylene) into each bore, it would quickly drain down from the No.6 piston and slower in 5 and 4 and not at all in 1,2, and 3.
Long story - short, I found broken compression rings on 4 of 6 pistons. No.6 had both broken in several pieces and the other 3 only the top ring. I checked all the pieces and they measured out to a 'width' of about 0.060" (compared to new ones at 0.078"), so the rings would 'flutter' and were doomed to fail. Yes, there was a 'ridge' in the stock bores- but not excessive. I elected to hone the cyls. and re-ring (plus bearings, gaskets and seals.
That was 5 years ago and car has run fine, during accumulated 6 thousand miles - - until a couple days ago. Seemingly out of nowhere, I noticed a miss below 1500, and yes, it is again in No.6. The plug and wire are good so I did more tests.
Compression this time showed ALL cylinders at 130 and leakdowns between 20 and 30 percent - no.6 at 20%.
Now the question - - - Do I have the same problem as before or is there something else to examine first? I had intended to do a proper rebuild in a few years, primarily a re-bore, with new pistons - but hoped for (and expected) the engine to run fine throughout.
Have any of you encountered this situation before? Thanks.