?? Pigeon on the antenna, moon passing overhead, passing lorry sucking away your power
Normally it's due to a dodgy connection, usually the patch lead, PL259 connectors can be problematic. Does the power
change if you wiggle stuff around, like moving the watt meter or the coax to the antenna. In a mobile setup quite often the coax gets pinched depending on how you routed it and causes problems, (especially if squeezed through the door sill).
Doesn't sound like much help I know, but usually best to get the obvious stuff out of the way.
Hope you get to the bottom of it.