A fully charged 6.0v receiver pack will normally peak out in the high 6.x to low 7.x voltage range. This is often the maximum amount of voltage that the receiver and servos can take and you may see servo jitters when you first power up a freshly charged receiver battery. If you let the plane sit with the radio system on for a minute or two, the voltage level will drop to the normal operating range and you will then be good to go.

Standing too close to your receiver with your transmitter antenna fully extended can sometimes oversaturate your receiver on 72Mhz systems and cause temporary jittering. This normally clears up if you take a couple of steps back from your plane with your transmitter in hand.

Test a little further and you'll like find one of these problems is the source of your trouble. Since you're flying with a 7C FASST transmitter and 617 receiver, it's most likely the former.

Remember that Futaba, Airtronics, and Hitec 2.4Ghz FHSS receivers all fly perfectly fine with 4.8v receiver packs. Only Spektrum/JR receivers require 6v receiver batteries to help prevent "brown out" or "lock out" conditions.