My 73 450 sl has off and on idled a little roughly in the year I've had it. I'm not sure what causes it to act differently from time to time. A week ago it was a little rough, and the last two days pretty smooth.

However, seemingly unrelated, within the last week there have been a couple of times when the rpms at idle have gotten so low the car has almost stalled. The idle seems relatively smooth, but it just drops to below 500 rpm. If I put it in neutral its ok, but it will almost stall if its in drive. The first time it happened it was unseasonably hot here in So. Cal., and I'd been driving in a lot of traffic for a couple of hours, and the car was relatively hot. This evening the outside temp and the engine temp were relatively cool, and although it wasn't as bad as a week ago, it still was acting as though it might stall at one stop light, although it was better at the next. With the heat, a friend suggested the problem was with the seasonal oxygenated gasoline, causing something similar to a vapor lock condition.

Its been about 18,000 miles since a tune up. My mechanic says if I get the car tuned up, particularly having the valves adjusted, it will run smooth.

Any thoughts on what the problems might be? Thanks.