1987 420 SEC idling/revving problem

My 1987 420 SEC has an intermittent revving/idling problem. It starts first time, but immediately revs to between 1500 rpm and above, before falling fairly slowly and erratically to about 500 RPM. (My old 300SE would go to about 800 RPM then drop smoothly and quickly to 500 RPM). Also, when doing about 30-40 mph in town, the revs can suddenly drop slightly.
The car has only done about 300 miles in the last year as it has been on a garage forecourt before I bought it recently. The dealer also allowed the fuel tank to run dry.
The fuel consumption does not appear to be too bad, and this problem does not seem to occur at high revs on the motorway(Freeway). The car has done 84,000 miles and was last serviced about 18 months ago at 82,000 miles.
Can anyone suggest any possible solutions to this problem?
Thanks in advance for any assistance.