"...half hour into the ride..."
This is classic "something embedded in the tire", and it can make you nuts (or more nuts...). If you still have a patched tube from this tire check the location (I assume you line up the label on the tire w/ the stem). Look real carefully inside the tire for small cuts or holes. Odds are good that there is a real small piece of glass in there; you might have to actually dig it out a bit.
__________________
Today, I believe my jurisdiction ends here...