You may have a lean condition causing backfiring through the carb. Hook up a tach and screw the idle screws in until the engine slows, then outward until you get a steady RPM reading and it won't go up any further. Then buy this: http://www.classicindustries.com/product/1968/firebird/parts/l582.html

The vacuum needle jumping tells me there's something wrong with your engine. Refer to this, it should help.
http://www.gregsengine.com/using-a-vacuum-gauge.html