mid rpm flat spot

im having a similar problem with my '90 gt. except mine stops at between 3000 and 3500 rpm. dont know what the problem is either

I had an ignition module on one of my gt's that was partially faulty and caused it to cut-out between 3500 and 4500 rpm but ran great before 3500 and after 4500,but you said you already changed the module,but there is always the chance of a new part bein bad.you could have it checked for free at autozone just to make sure its good eventhough its new.the pip inside the distributer could be bad but i wouldnt replace the whole dist.i would just buy the pip from autozone and install it yourself.its only $24 bucks which is alot cheaper than a $100 dollar dist.unless your factory rev limiter has gone back inside of your computer.it cuts out at 6250 rpm.