Still not enough information. Once the bike starts to act up, does it straighten out when you slow down? Will it run OK if you ride at lower RPM's? The tank has been on and off a number of times. All it takes is a fleck of junk to get into the float bowl to hang up the float valve. And the symptoms of a hung up float valve are very similar to what you describe. I'm at a bit of a disadvantage, as I can't hear, see, smell or feel what is transpiring, and is why I am not making any declarations of certainty about your problem. The diagnosis could run from a fleck from a disintegrating petcock packing in the float bowl to the valves having been set to the tight side. Tight valves generally create problems on a hot engine, not a cold one. I have no evidence what so ever that either is the issue. Your mechanic may have done an excellent job, and you are experiencing the problems that go with running an ethanol blend gasoline in your bike, that has separated.
We all have our own methods of trouble shooting. When problems seem like they may be fuel delivery related, I like to drain the bowl and start fresh there. Maybe two and even three times. It's like clearing the bench off before starting a new project. You have stated that you have pulled the tank repeatedly to trouble shoot, but didn't tell us the method or direction of your troubleshooting. My thoughts now lean towards returning the bike to the mechanic. I'd be paying attention to what he does to correct this issue, as it may well repeat itself somewhere in the Baja.
“many a trip continues long after movement in time and space have ceased”- Steinbeck, [I]"Travels with Charlie"
[FONT="Century Gothic"][I]Sometimes your only available transportation is a leap of faith[/I] [/FONT]