na man sounds like the dreaded 3800 rpm problem... usually happens between 3 to 4000 rpm but happens up in the higher rpm bands as well... i had the same problem, in exactly the same way... i tried to pass someone and it just dropped off. i read up on it and its an easy fix. the reason its doing this is becuase your secondary fuel injectors come on at around 4,000 rpm... mazda did a horribly bad job grounding the electrical system in the car... so when the secondary fuel injectors come on it pulls too much current with not enough ground. get some wire... 10 guage at a minimum, try to get bigger guage i.e., 8 guage, 6 guage...and so son... first put anoter ground from the negative post on your battery to a ground on the chassies... like a bolt or something... and then drive it... if it still does it... there are more grounds that need to be fixed... theres one on the intake manifold... on the exhuast... and on the block... also on 2 terminals of the computer and hell you could trace out the ground wires for the secondary injectors and just add another ground to them.... oh yeah... check your fuel filter too... it could be dirty