Nathan,
did you check the condition of the piston sleeve? The first production run (or two) of the 'ABC' (really ABN) OS longstrokes had very poorly plated sleeves. OS would replace the piston/sleeve for free if it was obviously a low time engine (and no 'bluing' from lean runs). I sent 2 back as did quite a few other pilots (including Ivan Kristensen who ran the OS RF-ABC pump briefly). Plugs and bearings would go 'bad' very quickly with all the nickle plating floating around inside the engine.
I hope new bearings solve your problem.
-Will