You need a diffeent meter for checking your battery pack. An ESV (Expanded Scale Voltmeter) puts a load on the pack, without it you really aren't learning anything with the meter. The minute you put a load on the pack the voltage will drop.

I'm guessing that 30 minutes of flight time has your receiver pack pretty close to the point where your receiver will quit, or start seeing glitches during the next 10 to 15 minutes of flight. ALWAYS do an overnight charge before you go to the field, and then test the pack with an ESV after no more than the 3rd flight (and every flight after).
Dennis-