5K Stumbles on Start

[Steve Sherman <spsherm@ibm.net>]

My wife's 5KCSTW has sort of caught a cold...

When one starts it up first thing in the AM, it catches just fine, but
stumbles along for a while.  If you take your foot off the gas it might
die and just stumbles along at ~500rpm or so.  If you race the engine up
to 2000rpm once or twice, it settles down into a good idle and seems
fine, even without the foot on the gas.

I can't quite figure what could be causing this.  Vacuum leak seems
unlikely because rev'ing it up a time or two fixes the problem.  Low
fuel pressure maybe?

Any BTDT on this problem?

TIA

Steve