Rugging despite constant acceleration – who knows advice ?

Hello forum, I have an Omega 2.2 BJ 99 with now 138 TKM. The engine was running at almost 90 TKM. Now I have the feeling when I speed up that there are “power holes” in the middle speed range. If I keep the accelerator constant and so accelerate from the lower range, then the thrust snaps again and again briefly, and it feels ruckle. That was never so before, but the Omega made a very pleasant impression. Has someone an idea what that could be. ? Inspection is already 25 TKM, will have to take him there soon, but maybe someone already has a tip on what that might be. Sorry, if this is a stupid question, I am absolutely lay… Thanks already ! Greeting Sascha