Jittery image on VGA projectors with MBP

Recently I have had trouble projecting presentations from my MacBook Pro (15", early 2011, OS 10.8.3, 8 GB RAM) to several different VGA projectors. I am using a mini-Display to VGA adapter and use 2-screen mode (not mirroring). Initially, everything is fine - the projected image looks clean, but after a while, lines start to zigzag and jitter - as if someone was physically bumping the projector (or like a bad TV signal). This gets progressively worse and makes it nearly impossible to give a professional presentation (which I am required to do for my job). I have bought a brand-new miniDisplay-VGA adapter - no improvement! I am very careful to make sure that connections and cables are all solid. The same problem does NOT exist for external monitors hooked up via DVI or HDMI adapters/cables. Any suggestions/ideas?