Do you have HO lamps that are rated 'CT'?? Not all HO lamps (8') are cold temp. Check the bulbsfor something like 'F96-T12-HO-CT' with a color temp somewhere in the mix.
Could be that the bulbs are not seated in the tombstones or sockets.
HO signs I have worked on in the past, have sockets mountedin punched holes in a sheetmetal channel on each side. Signs that are 'aged' may have weak springs in the sockets, or the frame that the sockets are mounted in may be slightly expanding causing bad contact at the sockets.
Did you notice any slight signs of arcing at the bulb ends, or on the socket contacts?
Last edited by HotLine1; 03/09/13 11:14 AM. Reason: added bulb data