Check the following:
- Are the probes or wires making good contact to the sample?
- Are the contact I-V characteristics linear?
- Is any contact much higher in resistance than the others?
- Do the voltages reach equilibrium quickly after current reversal (there will be some delay for semi-insulating GaAs)?
- Is there visible damage (cracks, especially around the contacts)?
- Is the sample in the dark?
- Is the sample temperature uniform?
- If there are large temperature gradients across the wiring, are dissimilar wiring materials being used?