2005 Silverado-V8 Flex Fuel 4X4 20,000 miles No PowerI

I've asked this question before,  still have the same problem.
A while ago my check engine light came on,  I had it checked at Autozone.  Code said
MAS was bad or another sensor could be the problem.  I waited a few months to
change the MAS because the light went off,  but eventually came back on,  and truck
was losing power under load.  I changed the sensor at this time.
The truck still has no power trying to pass, driving uphill,  and driving at elevation.
I took the truck to my local chevy dealer,  no codes showed up and they said nothing
was wrong.  They must not have gone for a test drive.  It's obvious something is wrong.  
The dealer did say the truck is running a little lean,  but they don't know why.(?)
My question is,  could it be the converters plugging up?  Since I did drive the truck a
few months before changing the MAS,  and the engine is running lean,  could this plug
the converters.  I just can't just start changing sensors.
The transmission seems to be shifting correctly and only has 20,000 miles,  I don't
think it could be the trans.
I've only used gasoline and not flex fuel.  This year truck has the in-tank,  100,000
mile(?)  fuel filter.
Any ideas out there?