XTR115 current loop transmitter more steady output
Check all your connections. Look at the output with an oscilloscope as well as a meter.
Connect a 10nF capacitor across the loop connections at the transmitter end.
Make sure your Vloop is a minimum of 7.5V + ~22mA * RL (RL is inclusive of your current meter input resistance).
Do not add a capacitor on Vref without implementing one of the compensation schemes described in the datasheet.