Get a vacuum gauge/fuel pressure gauge like this:
http://www.ebay.com/itm/VINTAGE-engine-c...=item25adbf6767

T it into the fuel line by the carb. Tape it to the outside of the windshield where you can see it, but it can't leak into the cockpit. Drive till the problem occurs, then see it the fuel pressure is excessively low or high.

Also, for a different test, hook the gauge to manifold vacuum, and see what it does when the problem occurs. it should be fairly steady, not jumping around. Also, if you can get your hands on an air/fuel ratio meter, like an LM1, you can see it it goes rich or lean when the problem occurs.