As many have noted, this is very likely related to a vacuum problem. I have experienced a similar problem on my 240d and 300d when changing the oil. When pulling the top off the oil filter canister, I would accidently pull the vacuum line loose from the shutoff valve. You mentioned the recent work on the A/C, I'm assuming this problem didn't exist previously. This could be as simple as a line pulled loose in the engine compartment. If you don't find a loose vacuum line, you could have a tech check out the shutoff valve on the injector pump, or check for other vacuum leaks in the system. Hope this helps.
[This message has been edited by bigbenzman (edited 09-25-2000).]