Does it do this when cutting, or when not cutting. Variable speed routers will do this. Whenn they start to cut, the exra load slows them down and they then speed up to try to keep the speed constant. If your not making continuous cuts, but a lot of short cuts, it'll be more pronounced. It's also more common at lower rpm's.
But, it sounds like your saying it didn't always do it, and/or it's getting worse? If that's the case, maybe there is a problem with the speed control. If there is a problem, you should be able to get it repaired at an authorized Makita service center.