One way to check for a fuel delivery problem (which it certainly sounds like to me) is to get up to about 4000 rpm in second gear, hold your speed for about five seconds then punch it. If you don't get the bog this time, it's time to check your fuel pump, filters and fuel lines.

Fuel delivery issues show up when the engine pulls for a long time at higher rpm, just like you get in the higher gears. You can do some nasty things to the engine if you don't fix a lean condition. Having the engine recover a few seconds after cutting out is a pretty definite sign of fuel starvation.

Just a quick note, a friend of mine rebuilt his carb and had some serious starvation problems after - he put the little carb filter in backwards! It barely had enough fuel to get to 50 mph!