The code 5 is an 'sro' code that says that the start switch (the one that clicks as the accel pedal is depressed) fails to close.

I assume that both problems happen all at once. If so, the likely cause is the keyswitch or related wiring. Both circuits share a common positive source.

The pos. source starts at the fuse, then keyswtch, then goes on to feed all the switches. It sounds to me that as the load increases the voltage drops.