ok , by golly ,i think i got it.
If the problem is pretty consistant at a certain speed it is more than likely a cruising speed,correct?
I would think the gas is fine or it would be consistant across the board for issues.
Since the 97 and up is common to be found with egr iacv carbon build up , i would suggest cleaning the egr as it is built up with carbon and is sticking causing the hesistation stalling sensation as it unsticks again.
try it and let me know, you can replace it but it is usually easy enough to clean.
btw do you have any hot restart issues?