An old bulb is the #1 cause for erratic light output. It should have a white
coating on the inside of the bulb. If the quartz has melted and distorted then there is something seriously wrong, though it may just be a blocked fan.
If the new bulb went out after 10-30 seconds (maybe longer) and/or the fuse blew then I would double and triple check you have the right bulb. Just because it matches the old one doesn't mean it is the right one, it is quite possible the old bulb was also the wrong type [the sort of problem that can give endless frustration].
Don't be afraid to call nuArc. Their support staff is very helpful and will aid you with used equipment. nuArc isn't what it used to be, what with computers doing all the pre-press work, and they are now a part of M&R Print, but I have heard the customer suppor is still the same high quality and 24-7.
The 26-1K is still in production, so nuArc will be familiar with the unit and will have lots of spare parts available if you need them.
The 26-1K is a reliable and popular unit that has been around for 30 years [I first saw one in '79, when I was designing their instant-start power supply] and there shouldn't be any great problem getting it humming along.