I get this exactly after sitting, usually from damp. Clears up after 20 minutes like you said. Your cause is possibly damp, and almost certainly ignition related. Definitely, on this 10 year old car that may not have been serviced many times before (v. low miles), go ahead and do the spark plugs & leads, rotor and cap too if you like. They are always the first stop for a misfire like yours.
Let us know how you get on
2.5-16v 1990 90,000m Astral Silver
2.0E 8v 1986 107,000m Black