c200 automatic no acceleration when I need it!

I've been driving my 1998 c200 classic automatic for 6 weeks now and have an intermittent problem. Pulling out into fast moving traffic from stationary or overtaking on a motorway at about 55mph and nothing
really happens! In the last case I was stuck alongside a lorry for about 1 minute until I managed to get up to 60mph. Didn't matter what I did; kick the accelerator down, move the shift over to 4, 3 or 2 (it has 4 positions as well as P,D,N,R). After this happens, when coming to a halt I can feel a pulsation under the brake pedal, which is still there in Park, when the whole car pulses slightly. I switch the engine off, start it up and the problem goes away. Most of the time the acceleration is very good, easily getting up to 85mph and pretty brisk from standstill. I'm not expecting it to accelerate like a sports car but when it's wrong it's definetely wrong, and this problem is not present all the time. Any ideas anyone?
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Suspect MAS - mass air sensor. It meters the engine's incoming air so the ECU knows how much fuel to inject. These are not cheap so shop a bit before committing to a new one.
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Great suggestion! A little bit of googling and a 4 aerosol of carburrettor cleaner to clean the MAS, and the performance is markedly different. There used to be a "gap" in the gear change just after pulling off which has gone and the acceleration is great at all speeds, doing more than I would expect. I've only had a few hours to thrash it around but if it stays like this I'll be very happy! Thanks for a great tip!
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Enjoy it while you can for it WILL return to its errant ways, eventually you'll have to pop for a new MAS.
I know this.
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Why will it go bad again? More dirt over a period of time or is it actually damaged? A new one is about 120 GBP or 50 GBP for a non branded one off ebay. Would the cheaper be a problem? Lots of questions I know but hopefully you won't mind answering! Thanks.
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My MAS experience: Check engine light ON, I read the stored codes "Fuel Trim" had dealer diagnose and reset Check Engine light. Then I very carefully cleaned the wires in the MAS. All OK - for about six - nine months when check engine light again lighted up. Cleaned it again and OK for a shorter time.
Finally bought a new Bosch MAS from an online parts supplier and installed it. DIY online cost about $220 vs. $445 dealer plus $100 to install theirs.
Engine ten years old and 40,000 miles.
So don't waste time, shop for the exact replacement and buy it when the MAS acts up again.
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