Perhaps its time to consider some professional help....
Sometimes environmental issues such as high humidity or chemical vapors can deteriorate things. I had some drain acid containers leak into the secondary containment tub I had them stored in and it rusted the living heck out of lots of stuff in my garden shed. Often times (as I noted in this same thread, but last year) ducting combustion air directly into the unit can alleviate lots of issues. I've seen condensation build-up in pressure sensor tubing cause havoc.