Inconsistent Gain with Instrumentation Amplifier
Power rails are connected to a +-15 V power supply, inputs of the
  first stage are connected to a function generator, with two channels,
  which inputs 2 180° out of phase sinusoidal signals and output is
  connected to an oscilloscope.


It's quite possible that the function generator outputs are floating with respect to 0 volts on your power supply. You could try putting a 10 k resistor from each input to power supply 0 volts (pin 1 on INA105). Amplifiers like this need to have input signals that are bounded by the common-mode input range of the front-end amplifiers (+/- 10 volts for an OPA2111).