912 ULS UNSTABLE RPM

Answering your questions:

We are using AVGAS;

The fuel return line is ok, even when I isolate the fuel return it shows the problem;

all the fuel hoses are exactly the same as all other similar aircrafts;

Thanks!

I'm sure it is unstable, you can feel the engine loosing power.

We just bought the aircraft, it has more than 1000 hours flying, but the previous owner didn't have any mechanical experience and cant give me the information about it.

the aircraft arrived for us with a lot of things to be corrected, even the RPM was using a wrong rate to determine the real prop speed, and with that information wrong, the propeller was adjusted with a wrong information and was a total mess.

The first start of the day, and the arcraft warms up, I can reach 5280rpm, but after a few minutes, it drops to 5250, and unstable, the low peak can reach 5120 rpm now.

I have talked to many mechanics here in Brazil and they told me I'm just worrying about it because I have a very good tachometer that shows it. If I was using the Rotax tach gauge I wouldn't notice that and they told me to keep flying the aircraft to see if it reach some dangerous lost of power or not. I'm not feeling comfortable with this but I don't see anything else that could cause this situation.