Zach,

I had the same surging problem as you some years ago on a ST-61 also.

Unscrew and remove the needle valve and inspect the O-ring. It should be inside the Needle's casing and underneath the O-ring should be a spring. If it looks like the O-ring has an off-center compression mark on it, remove the O-ring, massage it, and replace it. Make sure the spring is in first or you'll never get the O-ring out again. You might have to cajole the O-ring out of the casing with a pin or something.

When you get it running good, there's nothing like a ST-61.