oh yeah the rmx is a fun bike isnt it?? haha it seriously has a mind of its own and moodswings like a teenage girl!
but seriously, check your piston. which way is the arrow pointing? if its towards your intake you found your problem. I know it sounds dumb, but my friend did that once and it ran similar to what you are describing. (i personally didnt even know it would go together that way)
but if that isnt it, which it probably isnt, you could try your timing, i had my timing plate vibrate loose and it screwed over my timing. Also you may want to check your pipe and silencer, if there is any hole or significant ding in the silencer it will rob lots of power. I dont know why, but i replaced the silencer on my bike and it woke it up amazingly with an exact same replacement. I think my pipe/silencer joint pipe thingy was dented, as i put a smaller pipe on it to replace it and it ran terrible.
One more thing to try is check your oil.. Every time i put too much oil in the bike, it would run exactly as you state, and spew transmission oil out the powervalves and the overflow tube. i dont know what hte problem was, i replaced everything i could think of including the powervalves and cylinder and all the gaskets and bearings in the bottom end.
well good luck!