I think maybe you needle setting are way out after all that messing, reset. The LSN is key here! do as InitialD says and you should have a good low speed setting, know your ready to start dialling in the high end again.
Also check out your clutch! a badly setup clutch will commonly give you the impression that you engine is running out of tune, to much slip could lead to over heating as InitialD mention, to tight + to early engagement your engine will bog down making you think it has no bottom end.