I have an alternate theory, so I'll set it up for target practice. When you say "topped off" it reminds me of a case of fuel starvation I accidentally created by overfilling the tank (on a non-stovebolt). Once the fuel level rose past the vent tube in the filler neck, it created a vacuum in the tank the fuel pump couldn't overcome. Please note, I don't have a vent tube on my panel's filler neck and I have no idea if this scenario is possible with the p/u style fuel filler. That's my theory, you may fire at will.
edit: Next time it happens, try taking off the gas cap and see if it fixes the problem. When it doesn't,
then pull the fuel lines. :p
