I am not sure when this started, perhaps after installing the Dragonborn DLC: suddenly miscellaneous quests do not show up on the map. They are selected, but no markers on the map. When I go into the journal, select the miscellaneous quest, then hit X to show it on the map, it just moves the pointer to whereever I am!

All other quests show up on the map as usual, and pressing X when highlighting them takes me to their location. This is a MAJOR bug as I can't start or complete anything in the misc quest list now (this also happens when I add a new Miscellaneous quest!)

Help! This is game killing, especially since I am level 53 and I have more misc quests than "main" ones!

Did you also select the whole 'Miscellanenous' section? It has to be ticked to show individual quests. I often untick it by accident and then have the same problem as you do.