Sounds like a jet hole is clogged up=snowblowers and lawnmowers do that surging once in a while. Take out the jet(s) and find a piece of stranded copper wire and seperate off a piece that is about 4" long and poke it into the hole easily. Re-insert the jets back to 1 turn out from bottom (Turn in real easy no to ruin the seat or jet) Once it stops then back it out 1 turn and start it up and see what happens.