My E53 X5 3.0 Diesel 2003 was running fine until I tried to overtake someone and the engine spluttered and power dropped. Although it started, ran and revved up to 2000 rpm fine, anything above that spluttered, as if it hit a rev limiter. On mechanics advice I disconnected the MAF (Mass Air Flow sensor) connector and ran the car - it was massively better but this time not rev past 3,500 rpm. After re-connecting MAF I drove to the mechanic who plugged in his diagnostic machine and it came up with an air pressure problem and he thought it would be the MAF sensor at fault but cleared the codes and told me to drive it again and see if it re-occurred. Then within 15 miles it went back to not revving past 2,000rpm. I've looked online and I can get a cheap non-oem MAF sensor for cheaper than another go on the mechanics diagnostic.
Any thoughts on what might be the cause? What else I could check or whether it's worth spending £30 on cheap MAF sensor and see if that cures it?