915 is fuel pressure

Try changing the large main fuel filter. That seems to take care of my same issues on the 912iS.

Ok Thanks Roger, I was guessing it was that but seemed to happen in just the last 10 hours. Pressure gauges work good for telling us that we need to change a filter if that's the case.

Harlan,

Your fuel pressure should never be down at 36 PSI, the engine would be losing power if that were the case for more than a few seconds.  The Injected Rotax engines have a very accurate mechanical fuel pressure regulator that maintains the fuel pressure between 42-46 PSI.  However, this pressure is in reference to the manifold pressure. If you are using a standard "gauge pressure" sender that references ambient air pressure, then your fuel pressure will show the wide swings you are seeing.  In other words, the injectors need to maintain a fuel pressure of 42-46 PSI above manifold pressure to function properly, and that's what you want to monitor. This will still show you filter loading if placed in front of the filter, but it will be MUCH easier to make that determination. With a differential type sender, your pressure should always read within 1-2 PSI of the same (somewhere between 42-46 PSI), which is a very nice indication everything is working right before you depart.  

If I'm correct and you are using a gauge pressure sender, then your fuel pressure readings are just about useless. They will allow you to establish "how it usually looks", but that's about it. You need to first replace the sender with a differential type that has an air tube that runs up to the airbox for reference pressure. As a workaround, some flight displays do have settings that take manifold pressure into account and then give you a differential reading by calculation.  This works OK, but often does not properly compensate for altitude. I know both the Garmin G3X and the Dynon units have this capability. That being said, the best practice is using a differential pressure sender.  

I can't say if your filter is getting dirty (seems like Roger is always right), but assuming you are using a gauge pressure sender, then a change in ambient conditions or the throttle setting you are flying at would also cause this change.  

Plus if you want to make sure the hoses are clear  then when you remove the large main fuel filter put something to catch the fuel and turn the pump on for a few seconds. I haven't had a bad fuel pressure regulator, but you may have one. If all else fails maybe replacing that.