Hello, We've been using the INA103 for 25 years as current sense amplifiers in circuits similar to yours. Assuming you have the reference pin (7) tied to the common ground of your floating power supply and INA103 supply then a suggestion is to provide transient protection on the inputs. The Ap Notes suggest a 50 ohm resistor paralleled with a 1.2uH inductor in series with each input. We place PNP SOT23 transistors (lower leakage) to the supply rails. These are configured differently - collector to the input, base to +15V and base to the input, collector to -15V.
Turn on/turn off and load transients can result in brief but damaging over voltage events on the inputs of the INA103. This should eliminate one possible source of your problem.
