From my experience, the jittering motion is normal. With my 2 percs, the submissive one (smaller) did that quite a bit at the beginning, but now (2 years later) has died down to an infrequent occurance. Also watch for them to "lay" near the top of the tank on their sides at night...its pretty cool. My submissive perc also did not eat much the first few weeks, but is just fine, so for now I wouldn't worry.
Don in Ohio
125 FOWLR (sans fish!) aka "Tank of Doom"
20 Saltwater QT: 1 perc, 1 bangai cardinal, 1 crab, 1 sandsifting goby, 1 sandsifter star, 1 cleaner shrimp.
90 Malawi Cichlid and 1 "Beta Blob"