Throttle response: none, low, and normal

[joesdad]

My 2004 maxima has a problem with throttle response. We have replaced the throttle body four times, first three by mechanic with remanufacured, last time by me with new Hitachi. It always seems good for a little while, then reverts to same problems. Often, it runs normally, then out of the blue, it goes bad. It may give almost no response, where rpms never go above about 2500, even with pedal to floor. I can persist, and get it up to about 30 mph with VERY slow acceleration, shifting up when I get to a downhill segment, but it is very little power, just a little more than idle. It has a third mode, where it will rev up to about 4500 in first, then less as I shift up (it is a manual, m6). In this mode, I can get it up to about 45 or 50 mph, but ir takes about a mile. When in "normal" mode, it runs as ever, good power and throttle response. But, in the "no power" (a little more than idle) and "low power" modes, there is little response if I pump/flutter the pedal, though it does a lot more with the pedal on the floor than with less pedal. Obviously, it is dangerous in traffic if it is normal and then drops into one of the other modes.
The last time my mechanic had it he checked all the wires (some of which he had previously replaced), and he went through all of the Nissan-specified diagnostic protocols. Finally coming back to the throttle body. It was after this that I bought a new Hitachi online.
I also had it at the dealer, who I hold in fairly low regard. They said, "no problem, we'll just reprogram it." They "fixed it," I paid $150, got it back, same problem as ever. When I went back, they said, "it is your non-oem throttle body." (This was after second reman installed by mechanic.)
Obviously, I'm stumped and frustrated. Any ideas/tips would be much appreciated. THANKS.