Steet Legal WR400 engine chugging on bottom end

So heres the deal....I have a street legal WR400. She is my daily driver and its only three miles to work and Im either going 35 or 25 mph on the main stretch of road. Im in the Marine Corps and there is nothing better than waking up and driving to work on my 400 then enjoying a nice cup of coffee. I have a CRF450 and i ride it at the track, I do all sorts of my own maint such as air, oil,chain sproket, bearing, tire, im not a newb when it comes to diagnosing a problem. Anyways, here is my problem and it has really stumped me.

Im going down the road and im holding the throttle at a steady 25 or 35 mph.

-steady throttle control

-wheels balanced, sprokets are new

-chain is new and good to its not bouncing off the swing arm or guide from what i can tell when its on the stand

-oil is fresh

-perfectly smooth road

and then here is what happens....

the engine start chugging just below 4000 rpms and the it begins to rattle the whole bike.

And this is what weirds me out. I pull the clutch in just a bit, to take up the slack all the way down to the clutch lever. so at this point the the clutch is still fully engaged and then the chugging starts to go away and it becomes a smooth ride. *note* just because im holding the clutch lever there is no increase or slip in the clutch....someone please help. I put royal purple in it and until its time for an oil change im guessing its either clutch is about to go or maybe the with a 42 tooth rear sproket is unavoidable when it comes to the chain bouncing off the guide or rub block. To whom it may concern....whats could it be?

Are you sure its not the TPS (throttle position sensor)? They don't seem to like steady speeds, especially on the street. My 426 has a bit of a studder/surge when riding at a constant speed on the pavement (pease don't tell the law!!) I have heard disconnecting the TPS will get rid of most of it but I have yet to try it. Whats wierd is that my 450 does none of that and the TPS is still connected. If its not that than I would suspect you may have some clutch issues.

Ill disconnect the TSP before mondays oil change and give a ride around the block to see if it the problem still exists. If its not the TSP then ill inspect the clutch plates during the oil change. Ill try to shop a video.

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