If as described you're keeping your fermentation temperatures toward the lower end of the yeast's range it shouldn't be a fermentation issue. I'm talking about the temperature as measured on your fermenters. Still...if you're fermenting ales in the low 60s you shouldn't get "violent blow off" unless you're fermenting 5 gals in a 5 gal vessel.
The old adage, "if your water tastes good you can brew with it" is BS. I think you should focus on your water. Did I miss what your water source is? Do you treat for chlorine or chloramines? I think your best alternative for extract is distilled or RO water. I'd try that and see how it turns out. Could be your fix.
Oxidation is a possible culptit but after 5 years I think you'd have figured that out by now.