1972 Beetle Loses Power at Sustained High Speed / RPMs

Hi all.

Bear with me, I'm brand new to this.

I have a 1972 Beetle that I recently bought. It has run perfectly
until a few days ago when it stalled on the freeway. I was eventually
able to limp home, and I replaced the fuel filter, which looked like it
might be clogging, and cleaned the screen in the fuel pump. It ran
fine the next day until I was driving home and got on the freeway
again; it died in the exact same spot. This time I could not start it
and had to tow it home.

I replaced the air filter and opened up checked the carburator, which
seemed to be in normal working order (nothing obviously wrong with it,
anyway, but this is the first time I've looked at one, so what do I
know?). At this point it would start, but stall after a couple miles
and need to cool down in order to start again.

Finally, I broke down and took it to a mechanic. He found a bad valve
and fixed it, claiming that was the cause of my problem. He also did a
general tune up, replaced the plugs and points, and replaced the
condenser, which he said was the wrong part. The car was running great
when I left there.

Unfortunately, within a minute or two of getting on the freeway, it
stalled again. It was definitely better than before, and I was able to
get off the freeway and drive normally on surface streets to my
destination.

Basically what happens is I get up to speed (around 65-75 MPH) and ease
off the gas, at which point the engine seems to misfire a bit and lose
power in spurts. I respond by giving it more gas, and it loses power
altogether, and the engine dies as soon as I take it out of gear.
After sitting for a few minutes, it starts again but runs rough and
quickly dies again when the RPMs get high.

It seems like a carburator problem to me, but I am at a loss as to how
to correct it. Any ideas or suggestions would be greatly appreciated.

Thanks,
lukas