Is it perhaps just Anavasi kmz maps that are causing the problems, or is it something else?
Does your Oregon read any other kmz maps from its SD card? ?
(If you haven't got any other kmz maps, you can make a small one for testing eg with Mapc2Mapc (free trial, then £15 to buy).
Is your path to SD/Garmin/CustomMaps free from typing errors? (On my Oregon 450 the upper/lower case doesn't matter, but the exact lettering is crucial).