"The recent preponderance of problems has me wondering if the cause is duration of use exposing a problem or a correlation with heat/humidity exacerbating poor soldering/design."
When I called Onkyo, I point-blank asked if this was a common failure on the NR708, and they said "No, this is typically not a problem. We don't have a record of problems with these. It's just a warranty repair on YOUR unit." I knew this was crap because I had been to this forum first and read all the complaints from people with this exact problem who have
sent their units back to Onkyo!
Then they asked me if my receiver is enclosed in a cabinet because overheating can damage it, which would NOT be a warranty repair. Unbelievable!
No, my unit is NOT in a cabinet, and it doesn't have anything sitting on top of it. It gets airflow in pretty much every direction because it's sitting on a plank under an antique desk I converted into a media center. Thus, it is open all the way around the receiver. It gets plenty of air, and I clean it regularly because it's easy to do so with my setup. It's in a climate-controlled environment. It's not like I have it sitting outside on my covered porch in a metal box. Sheesh. Anyway, I came away with the impression that they were looking for ANY excuse to blame it on user mishandling so they wouldn't have to do a warranty repair.
I don't know if my experience with Onkyo's technical support is unique or not, but the whole exchange left me with a very sour taste in my mouth to the point that I wouldn't consider purchasing another Onkyo product. It's really too bad because I purchased Onkyo in the first place on a recommendation from a trusted friend who has one of their older receivers and LOVES it.