Fuel should last a lot longer than that. It doesn't have a very long shelf life, true, but a couple of months shouldn't be a problem.
Every time I've ever got a bad tank, it's been shortly after the petrol station has had a delivery. (Either I've seen the tanker leaving as I was pulling in, or else they've been closed while they were taking the delivery, and, since they were the only petrol station around, I've waited until the delivery was finished.) I think that, as the tanker pumps in hundreds of gallons of fuel at a pretty fast rate, it churns up the fuel that's in the tank, and all the crap which normally sits at the bottom of the tank gets mixed in with the fuel. Since I figured this out, I've always tried to avoid petrol stations which have just had a delivery, and I've never since had a bad tank. Could this be the cause in your case, I wonder?
I know that AvGas is regulated much closer than MoGas. I don't know if there are any rules in place to prevent this (e.g. cleaning out the storage tanks at AvGas retailers on regular basis) but I wouldn't be surprised if there were.
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