Vacuum testing question

[Chrispbrown36]

I have suspected a vacuum leak for a bit now and finally broke down and bought a gauge. 1987 Chevy V10 350 tbi auto. What she is doing: the idle seems ok but when I put her in reverse she idles way down and sometimes dies. It doesn't happen often but sometimes it will do the same thing when put in drive. Most of the time she runs great going down the road, every now and then she stumbles on take off if she hasn't ran for very long. Cruising down the road, if I let off the gas she will back fire every now and then.....not a major backfire, more than likely I am the only one that notices it. What I have done: new intake gasket, new distributor gasket, new idle air control valve, new map sensor, new valve cover gaskets, and new pcv valve.
When I hook the gauge up the needle jumps from the high end of the normal area to above the normal area by around 1 or 2 inches of mercury and then drops back into the normal area. The idle gets pretty rough when I disconnect the vacuum line (coming off the back of the intake going to the brake booster) to hook the gauge up, which probably accounts for the bouncing around, but should the idle get rough like that? Am I hooking the gauge up in the wrong spot?
Any help you guys can give me would be greatly appreciated.