No limiter as such so in theory you could run at 100% (unless it has changed on the latest LaserCut boards if fitted).
Upping the power could heat the tube up quick if the flow sensor is not working (as in not turning the tube off if no flow) which could cause it to stop (or worse kill the tube) so worth checking the water flow is ok just in case. I would expect you would see it cut for a small amount of time first though.
It sounds like the tube could be failing - use a mirror and look down into the output coupler and see if the mirror is ok (obviously with the machine off!), It may just need cleaning which you can do but be very careful how you clean it as any damage and it definitely is new tube time!!
Tubes always fail when it is the worst time possible and when HPC are not in the area for a couple of weeks!