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I think I found your problem. You're switching T1 (pin 12) for internal/external instead of T0 (pin 13). T1 must be grounded all the time to work properly and T0 is the interal/external program pin. That's why works on internal but not on externals.Try to cut the vias from pin 13, cut the trace from pin 12 and connect pin 12 to ground and 13 to the jumper/pullup resistor.