Engine Shutter 50-65MPH

In my situation when I give it a little gas between 50-65mph (4th gear)the thing shutters. This is more noticable while climbing small inclines whereas, it's not enough to downshift into 3rd gear. I can blow past the shutter by giving it more gas and it'll downshift. I'm also getting crappy gas mileage, 17-20mpg versus 24-27 I used to get. I've replaced the head gasket (it blew), IM gasket, plugs (.060 gap), wires, fuel pressure regulator, Fuel filter, 2 injectors (1&6). I'm now looking at either the MAP sensor or the O2 Sensor? I get no codes from my scanner. However, I did disconnect the electrical to the O2 Sensor and received the code for that and also a low voltage MAP sensor code which would make sense of my problem? Cleared them both after reconnecting the O2 Sensor connector and neither have come back. The thing still shutters. I may just go for it and replace both the O2 and MAP sensors. With my luck it'll be the x-mission, which is what I'm afraid of. 1997 Grand Am, 3.1L, 100,000 miles.
Does anyone have any other ideas? I appreciate it.

I bought some Lucas X-mission conditioner for possible slippage. After driving it for about 30 mins @ 65mph I got two codes-The MAP Sensor low voltage fault and #2 Cylinder misfire. I thought possibly that the MAP sensor may have had something to do with it since it does send voltage readings to the ECM and from that the ECM adjusts for ignition timing and air/fuel mixture. I thought possibly the timing may have caused the misfire or too lean a air/fuel mixture. So, I replaced the MAP Sensor and erased the codes. Drove it for about 10 mins @ 65mph and got the #2 Cylinder misfire again. At least I didn't get the MAP Sensor code. Kinda drives better in Limp-mode. Mileage really sucks though.

You see, about 2 months after replacing the head gasket and such listed above, #1 Fuel Injector crapped out. I replaced it. Even though so many have told me to just go and replace all six at the same time. I didn't cause they are 100.00 bucks a piece. About a month later #6 crapped out. There's 200.00 bucks so far. You'd think I'd leard and replace all 5. Nooooo, Mr. Hammer head thought I'm still 400.00 ahead of the game. Now, # 2 reports dead. This is the 3rd time I'm ripping this upper plenum off, not to mention the alternator brackets, gaskets, o-rings and other stuff. Now, damnit I'm replacing all 4.

After 100,000 and one goes bad, replace all of 'em. If my little shutter doesn't go away I'm driving this thing into the Pacific Ocean and buying a Harley.

I have what seems to be the same problem but it seems to shutter at 45-65MPH. have not done much have replaced the fuel pressure regulator. it seems to do it when its hot maybe after 15 min