My new desktop, Asus 8PH67 EVO, Sandy Bridge with a single green/purple PS/2 port (or maybe it's purple/green, I don't remember which is the PS and which is the 2 and I'm not looking to find out now), gave me strange behavior with my M15.
I got through at least one reload of the box (it arrived blank), maybe two, with no keyboard trouble, but at some point I noticed the keyboard wasn't working, and thus began a stretch of flakiness. I noticed it would work in BIOS, but once Windows booted (Server 2008 R2 SP1), no keyboard functionality and no LED *Lock action. Damned annoying, especially since I didn't have a USB keyboard handy. (I have a blue cube in a box but I've never needed it so I never thought of it.) I'd hit the reset button and maybe the keyboard would work.
But it seems fixed now. The fix? I bought a USB mini keyboard at the local geek store. It hasn't left its original packaging yet -- the M15 hasn't acted up since the USB keyboard arrived. Insufficient sample size, but so far it's working.
Actually, now that I think about it, it could be that it was flaky with the Lexmark but not the IBM Options (Jul-94 split cord, Dec-94 daisy chain cord respectively), because at some point I switched them. I should probably switch them back, because the Lexmark feels tired. Actually, I should probably dig up that switchbox that works with ThinkPads. (The numpad was never connected, btw.)
Sigh. I wish I had been more scientific, but it was a long week of multiple failures -- FedEx failed to deliver my new desktop, then as soon as I got my desktop my ThinkPad died, my desktop was grooving to the BSOD extended dance mix (Duran Duran would label it the "Night Version"), my car battery died, and my other car got a blowout.
Maybe I'll go back and run through my paltry PS/2 keyboard collection in more controlled fashion. Really, I do know how to do that, generally; it was just a really bad week.