how to stop O2 sensors going overvoltage?

Vauxhaul frontera 3.2 limited (latest shape). only when running on LPG, mostly in damp conditions but also when coming down from motorway speeds downhill off the throttle, O2 sensors go overvoltage causing ECU light to come on. have replaced air flow meter, had car to LPG tuners for check up, they reset mixtures as it was too rich and cheched presures and diaphram which were both ok. have checked out all LPG wirering under bonnet and have sealed all wires ect. car is well serviced and platinium plugs that are good for 40,000-60,000 miles changed every 20,000 as lpg runs hotter than petrol. when ECU light comes on, throttle feels like an on/off switch around 1500 RPM