The laser on my 40W started acting up after about 60-70 hours of occasional cutting. It would start to cut, and then stop cutting (I pretty much only vector cut). Turning down the 10-turn pot dial would trigger the laser to again switch on and start to (poorly) cut. Turn it back up to the 15mA and it would stop cutting again.
FSE was contacted and I was told it was either the laser tube or the power supply.
I still had the machine that was originally borked (details in another couple of threads here), and took the laser tube out and the power supply, which was an easy job.
I put the near-new laser tube from the "old" machine into my new machine first, in the hope that this would be the easy fix, as I was unsure whether it was the power supply that was the original problem for my first machine.
It has worked beautifully ever since.
FSE gave me some lark about only guaranteing the laser tube for two months and the manufacturers warranty only being good for two weeks on their 40W tubes, and said I'd have to get another tube from them. Luckily I still have the first machine that I'm slowly striping for parts for the faulty second machine that they sent.
I'd be looking on eBay for a 40W tube from a China supplier for a MUCH cheaper source of tubes than FSE.