Bill,
I had a loaner printer here for a while and had the same problem. It turns out it uses quite a bit of power in bursts, more than the external power supply can supply on it's own. I was trying to run it without batteries but it worked fine once I had both batteries and external power.
I presume shining a light on it (especially the sun) warmed the batteries enough to give an extra power kick.
--
Geoff Elder
http://www.SplitSecond.comhttp://www.SplitSecondRealTime.com