I wouldn't limit yourself to the intake manifold seal.
You can have a vacuum leak anywhere.
Further, it really could be the mass air sensor. I've read from technicians here (maybe even Stevebfl) that it's impossible to diagnose a bad mass air sensor. The only way to check is to take out the old one and put in a new one.
FWIW, they are about $225-$250, and you can install one in 15 minutes. On my car (different engine, 3.2 liter M104), it's in the big, fat plastic hose next to the air cleaner.
It could also be the idle control valve. I'm sure a real technician could diagnose whether you have a vacuum leak or a parts failure. If you have no diagnostic tools, you could always trouble shoot the problem yourself by throwing parts at it until you replace the right part, but then again, that could be expensive.
You can check for a leak at the intake manifold seal by spraying water at the gasket with the engine running. It will bubble if there's a leak. You can also check for hoses that are cracked or came unattached.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".