Stevejb wrote:...pH of 6. The starting SG was 1.060. It bubbled along nicely then on the third day it stuck. I checked the SG which had only dropped to 1.055 but the pH had plummeted to 3.5...
This is, to my mind, a direct result of having a Lacto infection (sometimes can be desired).
Lactobacillus will eat a small amount of sugar from your wash, and in return generate lactic acid, which contributes to the pH.
Very common to get that level of sourness in a sour beer like Berliner Weisse, even some Belgian Lambics will sport that acidity level.