Dear Phillip,
Had the same problem (3 weeks ago) with a Radiant/Sirrus Wallhung (non-condensing).
At first, thought it was a faulty gas solonoid until it was
a) tested with a continuity meter, and then
b) substituted/replaced.
In the end the main control PCB was replaced.
Working reliably ever since. rgds
Aiden Scanlon. Dublin, Ireland, March 2009