Are you using a NIMH battery pack? They have a tendency to drop voltage under load quite dramatically. A pack testing at 4.9V even under load from a volt meter could drop very quickly to 4.3 volts at 500 ma (an engine running up for take-off). That drop is not instantaneous, so you can be fooled by a quick (2-3 second) voltage check. If you can, try and simulate the load your system will put on a battery pack. This is easy to do with an ignition pack as you know the maximum likely load (from the manufacturer). Then keep the voltage meter attached for 10-20 seconds and study the voltage drop. I suspect that if you had done that, you would not be repairing a model right now.