Engine sputtering at about 65

LOL... I initially read that last line as "perfect for gas tank explosion"...

Just keep in mind that as your speeds increases, the fuel consumption increases as the wind resistance increases. So low speed is less load and less fuel flow than at cruising speed, even at the same RPM's with a different gear. And sometimes the plugging up takes some time to show itself; the crud can fall off the sock when you stop and then build up again. It really does happen this way... and the best filter in the world won't change what is going on in the tank.

If you're gonna do the tank anyway, I'd do it now to get this out of the way. Then look into the carb again to see if things were plugged, and check the fule pressure at idle.

Fuel pump crapping out is certainly a possibility too.