86 GT having problems...

Hello all, I'm new to this forum but I'll dispense with the pleasantries and get right down to the problem I'm having.  I acquired an '86 GT 5.0 from an uncle about a year ago, who until giving the car to me had allowed it to sit for about 5 years, barely covered, in a field next to his house.  After sitting for all that time it looked much the worse for wear but all it took to get it started was bleeding the brakes, airing the tires and charging the battery.  It drove the 75 miles back to my house without so much as a hitch!  After sitting at my house for about a month it refused to start anymore, so after much testing and a diagnostics check at the local mustang shop we decided to replace the corroded distributor, ignition coil, and ignition solenoid.  I also replaced the battery, plugs, plug wires, oil, a new fuel filter, flushed out the old gas tank and filled it with premium and a bottle of injector cleaner and finally cleaned out the intake to make sure the 5.0 could breath,.  After adjusting the timing it started right up and idles perfect.  But when I tired to take it for a test drive around the block, it has absolutely no get up and go, it starts to stutter and hesitate and backfire repeatedly under load.  I assumed that after sitting so long it may be an exhaust blockage so I removed the entire exhaust below the 02 sensors and sure enough one of the cats had completely fallen apart and was as far as I know clogging up the tube somewhere further back.  I tried driving it without the exhaust and it still seems to be having the same problems though, hesitating and backfiring under load.  Sorry to be so long winded but I wanted anyone reading this to know the car's prior history in an effort to help diagnose my problem.  Any help is greatly appreciated!  Thanks

-Ross  

Did you set the timing correctly when you replaced the distributor ?  there is a spout connector pigtail near the TFI connector that you must remove in order to set the base timing correctly.  It sounds like the timing is too retarded.  If you're going to keep running premium fuel, set the base timing at 14-16 degrees.  Keep it at the stock 10 degrees if you go to 87 octane. Hope this helps.  

I will try this afternoon and get back to you, thank you so much!  

if u are still having problems check the timing chain as if it is stock it may have jumped time  
we will all have our day in court (tickets suck)

the timing is out. you replaced the dist.. odds are you put it back in a little bit out.