Sounds like the LED bulbs were overheating and shutting down.
Mine used 921s, too. I replaced them with LED panels from Dealextreme (www.dx.com). I got warm white ones, which are much more comfortable to my eyes that the more blueish standard white ones. I haven't had any problems with them other than they're a bit fussy getting them plugged into the socket.
They have small and large panels. The small put out a bit less light than the 921s, the larger panels put out the same or maybe a bit more.
The power consumption varies a bit between units, but this is what I measured for a couple. Lumen spec is what they claim, and seems about right. 921s consumes about 1.25A each, so these use like 1/3-1/6 the power.
size partnum draw lumens
----- ------- ------- ------
small
283830 ~200 mA 200 lm
large
283831 ~360 mA 400 lm