Sometimes I wonder if dropping a tooth on the drive sprocket would help, at least with the 5-25 MPH range.  I have an MX400B that of course is geared way differently, that is a smooth as glass on the bottom end.  Come to think of it, my DT1C and DT175E do as well.  

Don’t let me discourage you from trying the drive sprocket change, but my 400 has a huge rear sprocket which gives similar results. It still has the bucking and chugging problem.

I did go up a little on the pilot jet for my DT400C, it helped some so I'll probably try going up a bit more.

Actually someone reported that going UP on the front sprocket helped.  His theory, if I remember correctly, was that the taller gearing put more load on the engine during cruising which smoothed out the surging.