I am actually running BFRPG for a few months now... It has helped my burnout a little (prep has caused far worse burnout effects when I still used 3.0E), but I still run into this problem...
I had imagined that you are suffering from DM burnout because DMing has seemed to become work. That is a common cause, especially with a prep-intensive ruleset.
Another possibility, however, is that you don't know "what you want to say". This assumes that every creative work, DMing included, has an element of communicating ideas. Sometimes, if we haven't been active enough in the world, the well of ideas runs dry.
If this makes any sense to you, you might understand what I will suggest next: Go hiking, go caving, do something out of the ordinary for you, walk around the seashore, go through the city at night.
Do anything that leaves you brimming with ideas you want to share.
Let someone else run a short campaign while you do this, and while you let your experiences turn over in your mind.
And especially, especially, especially take a break from the InterWeb, where a whole lot of folks are posting ideas, maps, and whatever else that might make yours seem pale by comparison (even while the same folks look at your work in envy).
The InterWeb is a bad, bad place as far as feeling good about yourself and your own work goes.
Oh yeah, and read any Gygax you can get your hands on, starting with the player's advice in the back of the 1e PHB and anything at random from the 1e DMG. If you decide to run a module, pick a Gygax module. There is no prose in the world like Gary's for re-energizing DM batteries.
Best of luck.
RC