Idle Surge/Stall

[spinedoc2304]

Hello to all and Happy New Year!

My 1991 300E with 105K miles on it has suddenly developed a problem. Every now and then, at idle, be it in drive or park, the engine revs up approx. 300-400 rpms and then drops down. 5-10 seconds later, it'll do it again. This may happen 5-10 times and then stop for the rest of the day. If I shut down the car, it may not do it the rest of the day. It has gone 3-4 days without it happening. I find that I really have to keep my foot pressed on the brake at stop lights because the car wants to surge forward and there have been times where I put the car in neutral when it is happening and it will surge and stall.
I've been trying to change out my cruise control amplifier with one from a junkyard and it seems like when I put the junkie in is when the problem started, although I can't be sure. Can this have anything to do with it?
I've read that it can be a failing mass air flow sensor. Where exatly is that so I can get in there and clean it with carb cleaner?
Any suggestions would be appreciated.

Nick