Well with an SVT manifold you have lots of places where you can have vacuum leaks. There is a vacuum line diagram under the hood on the VECI (vehicle emissions control information) sticker. Most vacuum will come off of the throttle body, but a few come directly off of the intake manifold- like the PCV and brake booster. A rusty brake booster will cause a very annoying vacuum leak coupled with weak/hard brakes. A bad check valve- the 90 degree connector where the hose goes to the booster- will do the same thing.
Trace down each vacuum line using the VECI diagram to help you locate them. You should have about 5. The longest one will be the one that goes to the EVAP system. It should connect to the TB, then go to the purge valve, and from there to the carbon canister. That one won't be it unless it's not hooked up at all- or the purge valve is stuck open. I'd give a stuck purge valve a 10% likelihood. It's really uncommon. The most common issue is the PCV line, but I'm not sure where it connects on an SVT manifold- if there is one at all. Check your VECI, and then look for what is on there.
1Turbofocus is right, the MAF is not your problem, what's happening with the MAF is a result of another problem. Stop throwing parts at the car, and work from the cheapest fix up, or diagnose it properly. We can help you with diagnostics- one of which is listening for the hissing sound indicating a vacuum leak. The best way to figure that one out is to simply remove each line, inspect visually for dry rotting or damage, replace if needed, and re-install. I have stopped recommending starting fluid or WD40 sprayed around the inside of the engine compartment to help find vacuum leaks because of the dangers involved- and it's a subtle change in rpm that most people will miss. In the end you just have a lot of flammables in a confined space without much new information.
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