Measure DVM functions returns 0

Hello,

My voltage measurement VI returns 0 every 200-500 calls. This function is used to measure voltage on a shunt. When I put a breakpoint when that happens I can see on the DVM front panel (Agilent 34411A) a different voltage value. Also as it measures voltage on a shunt there's always some noise if if there's no current flow, meaning some millivolts, never 0.

Any ideas will be appreciated.

Thanks
CT