Thanks for the reply.
Yea, it doesn't happen all the time, but usually hangs around for a few weeks and then goes back to normal. Last time it took a month, and it drove me crazy! I try to avoid exporting maps because you end up with the ESRI URL reference sentence at the bottom of the map. I avoid snipping because the quality is low, especially when you resize the map.
Attached is a sample mxd and a map layer. I made a new map with just the one layer, still having the same issue, so the bug is in there somewhere!