GPSRs work best on a clear day when you are out in the open and any available satellites are spread out across the sky rather than bunched up overhead. Of course, this is a pretty rare circumstance. . .
A pretty common interference problem in geocaching is tree cover. Since the woods offer great hiding spots, you are bound to be dealing with tree cover now and again. Thick canopy and dense trees contribute to interference.
You can find information on solar storms at NOAA's Space Environment Center website. Solar storms degrade the accuracy of your GPSr reported coordinates. The coordinates reported by your GPSr depend on accurate time signals coming from the satellites. Solar storms increase atmospheric drag, slowing down the signals coming from the satellites.