Okay, truck is a `99 F150 4x4 SC with the 5.4 and 38,000 miles. I installed a K&N air filter in January, and also modified the intake system by smoothing all the rough edges, polishing the throttle body "throat" and drilling approximately 30- 1/2" holes in the filter housing to allow air to enter the filter directly. About 3 weeks ago, I started noticing a little "stumble" or miss at about 1400-1800 RPMs. I had a Flowmaster 40 Series muffler installed a couple of days later, and the problem kept getting worse.
It progressed just to the point of the engine stumbling and surging at those RPM ranges. Check engine light never came on. I tried fuel injection cleaner and premium, thinking it was maybe some bad gas, even unhooked the battery cables to re-set the computer. Didn't help.
I finally took it to an independant shop last Thursday and they said that the MAF sensor was dirty and was reading wrong. He sprayed it with some carb cleaner and charged me $80. The guy also said that it was dirty because of the K&N filter getting oil on it. He said that it would probably be fine with the K&N in it for a few months, then I could just clean the MAF again when it started acting up.
Well, it started acting up again the next day. I couldn't get it back to the shop until today. On Sunday, I inspected the MAF sensor wires and they appeared clean and shiny. Sprayed them with carb cleaner anyway and put the factory air filter back in. Problem came back until the truck warmed up and it ran fine while driving around town, then I shut it off to wash it, when I restarted it, it was back again, and has been acting up since then.
I took it back to the shop and he hooked it up to the machine and said that the MAF "thought" that the altitude was 5000 ft. when we are actually at 2000 ft. Tech said that this would cause the truck to run lean and cause my problems. He also thought that the MAF sensor was probably bad and that I should put in a new one.
Does all this sound reasonable? I don't want to spend $170 for a new sensor if it isn't the problem.
Could the extra airflow into the motor be screwing up the computer? Is it common that MAF sensors go bad at 38,000 miles? Should I put my K&N back in?
Any thoughts into this problem?

 

I could be wrong on this so if there's a "guru" please correct me.

Isn't the wire sensor in the MAF slightly heated? If so even though it's clean it is entirely possible that it is defective or the heat circuit is defective, etc.

If you do have to replace it then see if you can find a larger MAF but you may have to reprogram the computer.

 

What is the advantage of a larger MAF? The Ford parts guy said that the $170 included both the sensor itself, and the section of intake tube where it sits. Is the intake tube prtion larger to let more air flow in?

 

I think the idea behind the larger MAF is that it will allow the engine to take advantage of the higher volumes of air. I've read some articles where it seems to be a good mod . . . I don't think it's cheap though.

I think the parts guy means that the sensor actually comes with the housing and when you take your old one out, you return the old housing for a credit. That's what happened when I recently replaced mine. I guess they re-use the housing and just replace the actual MAF sensor . . . thus the housing gets "re-cycled". I'm pretty sure the part you are getting from Ford is the OE part and not a performance MAF.

It's easy to change BTW . . . definitely a do it yourself. I've not that "mechanical" and I had no problems whatsoever.