Have you checked for a CLOGGED Exhaust System?
It will be Noisy, but you can disconnect it at the down tube, test drive down the street,
and SEE if it's the same. If it changes, replace muffler or Cat, and the Cat would be
my first guess, as it's most likely Orig.
The above is just something to eliminate that doesn't cost $$$$, but can a will cause
the issue you describe.