Hi John. I have to admit to having the exact same symptoms as you on my 940T, bugged me for a few years, but didn’t figure it out until just recently. I figured it had to be on the ignition side because it so quickly disappeared with a few good blips of the throttle after starting. I originally assumed it was slightly fouled spark plugs as things noticeably improved once the engine got hot or after a plug change. A new distributor cap and rotor also seemed to help, but with time I knew things weren’t perfect. Even a new set of plug wires only seemed to make for a temp improvement. Turns out I was chasing the right dog, just not identifying the lead hound. I had used some kind of generic aftermarket distributor wire, but it apparently didn’t have a good positive grip over the knurled stems in the distributor cap. Putting in a new Bougicord distributor wire fixed it instantly. Not that I’m saying that's your exact problem, just that it’s more than likely a less than positive connection on the ignition side. Hope that helps.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now

This may be a guess but I've had low idle situations upon morning start up but clears off when driving. In my case it turned out to be the alternator exciter wire was broken at its terminal. Haven't really paying attention to any missing dashboard lights every so often. That's life...The car was driven off with batt voltage until residual magnetism in the rotor self-excites the alternator to produce output at higher rpms. So my idle issue was due to low system voltage.

Amarin.