It's sure sounding like a power issue, why run the voltage at 5.5? I'm ALWAYS leary of BEC's that are not switching and I do not know if this one is but I think it's linear.
That said, after making sure it's not an errant tx signal and you think the battery/BEC are up to snuff, I'd be changing out servo leads especially if they've been there for some time and/or are not gold plated pins with a decent wire size.