Your voltage readings are too low. Don't know what you're using to check them, but if it's not an expanded Scale Voltmeter, get one.

A freshly charged receiver pack will often show as high as 5.5, but 5.3 is good. I never fly below 4.8 volts. The transmitter fresh off the charger should read about 11.0, but will drop to 10.9 or 10.8 very quickly (within a minute or so). Again, at about 10 volts, I start thinking "charge it".

Although two hours of "on" time with a 600 mAH receiver pack is theoretically possible, check the pack with the voltmeter after the second flight, and after every flight from then on. It's not worth losing a plane because you "thought" there should be plenty of charge left. [&o]
Dennis-