[MikeMa]

What diameter bit were you using and how deep of a cut?

 

[MikeMa]

A simple straight bit 8mm, from a set distributed by Bosch I bought many years ago, on full speed cutting in oak! That said, I dont think its a proper Bosch quality, but something cheap chintoc, and maybe thats why it struggles - the bit is not sharp as required?

It could be you are trying to make to deep of a cut in single pass. Anything that is cutting wood needs an opportunity to clear way the chips and dust that are created. With smaller bits, the spacing between cutters gets tighter, causing them to get clogged easily. Making a cut too quickly or too much material being removed can result in the cutter getting clogged causing even the most powerful router to have issues, or even resulting in a broken bit. With my 1/4" (~6mm) spiral bit I try to make each pass at 1/8" (~3mm) or 3/16" (~5mm).