Didn't I tell you guys? The problem went away, it hasn't happened again ever since I went back to the stock bumper.

Also FYI when I asked around to other car buffs, bumper was one of the suggestions I kept hearing. So even if it sounds strange, it appears to make a difference.

You also have to remember how hot Phoenix summers are. You can go on all day long about humidity this and humidity that, but the engine doesn't care about that. Not very many places in the US will get as hot as it gets here in metro-Phoenix area. I say this because I was able to travel with the aftermarket bumper on and not have any issues when I was traveling through California, Utah, and Nevada. It just wasn't quite as hot outside when I was going up those uphill highways, it makes a difference believe it or not.