I load test my batteries with a 2A load using the Hanger 9 volt meter. I'll get 4.2 to 4.6 volt reading on a good pack with a 2A load. I do fly all my planes including my 30% Extra and 33% CAP on Spektrum. The T34 you have would not have caused a battery voltage drop enough to cause your receiver to have a issue unless the pack was bad. Anything below 4.2 under a 2A load I consider unsafe to fly.

Voltage drop in accordance with load is a real science. Some battery technologies work better than others when you have a high load. I run 4.8 on all my large birds but I use SubC Nicad packs at 1400 MH. I use two of them for the receiver running Hitec digital high torque servos. Nicads allow for a higher discharge rate with a lower voltage drop than your typical NMH packs. Its not necessarily the size of the pack that is important but rather the max discharge rates at a particular voltage that is of concern. I avoid the LiPo's due to just adding another failure point with the addition of regulators.

If you have some time check out Hangtime Hobbies web site and refer to their battery section. Some very good information on that site regarding loads.