The '90 SSE (w/'91Park Avenue L27 longblock) developed a weird miss about a month ago. It would start and idle fine, but under low RPM load it would miss. Shifting down a gear would make it go away. But, that'* not what you really want to do when cruising down the highway at 65 MPH. It would kind of stumble then recover, and if you let it go on without shifting down, it would start doing it in rapid succession. But, it would go away at higher RPMs. Occasionally on some of the harder stumbles, the tach needle would jump down as well, which indicated to me it was an ignition problem rather than a fuel problem. Throughout, no SES light came on.
I pulled the plugs expecting to find some fouled, because it does use more oil than it should (quart every 1500 miles or so - I think the engine sat outside open before I got it). But, they were okay - not new, but not at all fouled. I had another DIS module and coil pack sitting around from a previous hard start problem on the '91 Bonneville (turned out to be a bad fuel pump - nothing wrong with the ignition), so I put it on to see if that made any difference - it did, it had a steady miss on one cylinder at idle.
I put the original ignition back on and got some new plugs & wires, since the wires had over 100k miles on them. Sure enough, although most had 4-5k ohm resistance, one didn't even make the ohmmeter needle move. Now with the new plugs and wires, no more miss.
Still find it weird that it didn't miss at idle or higher RPMs.