use propane and a small hose to check if rpm increases around elbows and throttle body to check for vacumn leaks. I it passes this test then before you buy the 180.00 MAF sensor- remove and clean sensor with electrical contact cleaner.(you will see sensor hanging in the MAF body) I recently had same problem on my 98 v70 and the code has not come back in 2 months since I cleaned it. Prior to this it would activate the MIL light pretty quickly after resetting it. I read about cleaning it on the Ford board.(apparrently the Hitachi sensor they use is very sensitive to contamination) Certainly worth the try. By the way when removing sensor the "hairdryer" is a must to make them come apart. Just takes a couple minutes on high!

good luck