Ste RB5138

I went to go to work this morning, it was probably around freezing tempature, the RB5 had been standing about 4 days outside.
Turned it over a couple of times as I usually do when the engine is cold, engine started and idled at 1500 rpm as usual for a couple of minutes, there was steam of the s/s headers which burns off moisture as the engine warms.
Then the rpm started to drop very slowly till it cut out? I probably should have blipped the throttle but I never have done before.
I started it again and it was fine, warmed up and settled at 800rpm, drove about 3 mile and back with no problems, I didnt take it over 3,000 rpm in case its the MAF again.

Any ideas?