Suddenly a shaker

This morning my iron head GT40 crate motor with an E303 that has given me 75,000 trouble free miles suddenly has a shake or shudder. Idle is slightly worse than normal but not terribly so, but there's a bad shake as it passes through 1,000 rpm. It smoothes out, but an extra vibration can be felt at all rpm. It must be the motor and not the drivetrain, since it shows the shake when revved in neutral at standstill. Code reader came up clean engine on and off. Plus, the EECIV cylinder balance test gave me 9 beeps -- all clear on the injectors. I did hear a barely audible grinding noise from my clutch -- a centerforce dual friction with over 80,000 miles -- on the first couple of throws this morning, but then it disappeared. Car otherwise starts, revs, shifts OK. Oil is clean and temp is low -- not likely a head gasket -- and I saw no evidence of an ignition problem -- an old MSD6A with relatively recent (20,000 miles) Ford Motorsport 9mm wires, Motorcraft plugs, and the good cap & rotor. Any thoughts on what could suddenly cause such a shake or shudder?

I haven't looked at the balancer closely, but it was new on the crate engine, although that's going back maybe 7 years now. But it certainly is not a 15 year old dried up piece. I was wondering if the centerforce clutch could have thrown a weight? It really doesn't feel like an ignition miss, since my experience is that they tend to be uniform across the rpm band, and even worse at higher rpm. This one has a definite resonance at 1000 rpm, then smoothes out as they climb, although there is still something going on there. But I guess that plugs and wires are first as they are easiest. I have an old cap with Motorsport wires still attached that had no apparent problems when I swapped them out, so a test changeover should not be too hard. I am also wondering about my idle air bypass, which I just cleaned out. I had the A/C recharged, and didn't want them messing with the idle setting, so I douched out the bypass to insure a steady idle for the recharge, and they did it with no apparent problems; everything was smooth and groovy when I got it back. A day later -- this. But I don't think it could be the bypass, since a vacuum-type leak would lead to a soft/high idle, and not likely have the mechanical-type shudder and shake I seem to be experiencing. Anybody have a clutch problem like this?