tribaltalon

its a 98 z28 ls1 with a nitrous cam around .581 lift. when it idles for a while, it gets into a loping pattern and you can hear a clunking noise but it only does it once or twice and then stops until the pattern repeats. my friends think its a motor mount or transmission mount. the motor does move around quite a bit when it gets to the middle of the loping pattern. theres no knocking/clunking when i start moving, and it revs good and clean. probably not a rod or anything right? are motor mount/transmission mount failures common with these cars? it is a 6 speed by the way with 82k miles. any other ideas of what to check?