RE: Let me get this straight ! [8|]
Regulated 6V is certainly not too much for the receivers. Most will tolerate unregultaed 6V packs (peak of 7.1V), although some will twitch at peak voltage.
I was confused, though, about using the 9Z and 8U. Did you say you are getting the same glitches with both TXs (that would seem to eliminate the TX, unless you are using the same frequency module- that could be an issue)?
Another failure point to isolate would be the TX battery pack. Swap it out and re-test. I've never heard of a TX battery issue causing an erratic signal, but I suppose it's possible.