Something happened suddenly it seems. You were driving, things in high gear were ok, then something happened and now you can wind out in low gears but everything goes flat in high gears. Rusted parts inside your muffler could have fallen off while you were driving and formed a dam in there. I've seen that happen. Sudden vacuum loss is a possibility. Failed coil is a possibility, but if you're ok in lower gears it isn't a very large probability. I suppose a failed/failing gas cap is a possibility (must be vented correctly or at a point the pump can't suck enough gas)...but sort of a stretch since that would not happen suddenly.
Jon, "Rochester Bs fail over several years"? That's about the nicest and most optimistic thing I think I've heard you say about that carburetor.
