Have you been lubing the chain?
500 miles in crappy, rainy weather with added salt from the roads can cause a sticky link in a chain if it's not looked after, or if it's left sitting still for periods of time.
The other thing to check is rear wheel alignment, if it's slightly not straight that will cause lumpiness in the chain as well as it tries to correct itself.
Check it again and make sure the rear axle nut is correctly torqued up and the adjusters are done up properly.
500 miles in crappy, rainy weather with added salt from the roads can cause a sticky link in a chain if it's not looked after, or if it's left sitting still for periods of time.
The other thing to check is rear wheel alignment, if it's slightly not straight that will cause lumpiness in the chain as well as it tries to correct itself.
Check it again and make sure the rear axle nut is correctly torqued up and the adjusters are done up properly.
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