Drivability problems

[meknized]

Hello

Hi, I am new here so hello everyone.
I have on nasty problem with my 89 GT. The problem is drivability. anything below 2000 rpm it fumblebums all over the place in any gear and the engine pulsates at any RPM whether through town or highway. I have had the car for about 3 years and this my 5th GT. I have also owned in the past 87 Gt conv. 92 GT, 89 GT, 90 GT and now another 89 GT blue and silver tutone with mach 1 hood and saleen wing.
First I'll desribe the mods. I have a coast 331 with slighty ported and milled GT-40X heads, FMS X-303 cam, 1.6 roller rockers, cobra intake with bottom port matched, 70mm TB,Mac intake tube, Accufab fuel regulator set at 40psi, 24lb. injectors, FMS cobra computer instead of A9L it's XZ3 with 75 mm bullet MAF, Accel 300+ wires, MSD 6AL, and coil, ASP Under drive pulleys, 255 fuel pump, flowtech equal length headers, X pipe, flowmasters, Tremec TR3550 with pro 5.0 shifter, FMS aluminum drive shaft, Moser 31s axles with 3.73 gears and auburn pro series differential, lakewood upper conrol arms and Megabite jr. lowers, cross subframes, plus a bunch of other little stuff.
If I haven't bored you yet here is some back ground. I have about 3,500 miles on this new combo. When I first got it running about a year ago I felt like this car was the fastest thing on wheels, put you in the seat hard, I was pretty happy with it. Even more so when I almost beat my friends 1998 Toyota twin turbo with mods. His best time is 12 flat @119 I was ahead of him until the top of fourth when he got by me. (we were at a street race Shhh) That is how well it ran at first, but a couple of months into driving it around. It started this stumbling around and pulsating feeling while holding the gas steady and has felt like there is a miss some where and has half the power when it does this. The kicker is that it is intermintent. I can go a night running perfect, the very next moring it'll feel like the engine is going to die on the highway just cruising and runs like dog S@%# This is so frustarating I hardly drive it anymore. I have checked everything I can think of and replaced alot of stuff like o2 sensors, EGR, fuel pump, wires, plugs, coil, I have had no codes at all show up. What dosen't make sense is that it didn't run like this at first now it does. ANY IDEAS WOULD BE GREATLY APPRECIATED. Maybe MAP sensor? I tired of playing parts replacement musical chairs. Sorry for the length of this. Hopefully you can help