car dies and idles hard need help asap!

so here's the deal i just purchased a 1995 mustang gt 5.0. It's my first mustang i've ever owned and am happy that i bought it. but i just recently did a tune up on the car with a new distributor cap and rotor, new plug wires, new plugs which are gapped correctly, new maf, new iac. and the car now goes over the 3000 rpm limit that it was having trouble going over before but now the car dies after it gets warm and when driving down the road. Basically it dies when i take it out of gear and put it in neutral and hit the brakes.... could this be a vacuum issue? or could it be something else? o and i've replaced the o2 sensors already. i bought the car as a project car and now i'm running into some problems. please add any advice if you can think of what this might be. thanks for any help i appreciate it.

yea i changed the air filter and the fuel filter the gauges the guy that i bought the car from had installed gauges on the car before i purchased it so i can see my fuel is sitting pretty level and my oil pressure will eventuall drop below 20 when it's idling and not in gear. i replaced all the parts because the guy said it would need a tune up soon and the deeper i dug the more stuff i found wrong, the maf was causing the computer to not let the car go above the 3k line on the tach, and now when i drive the car for a lengthy period of time it started dieing when the brake pedal was touched, and thanks for the info i'll look into buying the scan tools i need. like i said this is a project car and i needed a start point to get it running right first. so thanks for the info about the scan tool i need one obv. but as of right now it's been a replace and eliminate strat. which has been draining lol, but at least i know those parts should be good now. the iac, maf, distrubitor cap and rotor, wires, plugs, fuel filter, 02 sensors shouldn't give me problems. but i'll look into buying a scanner tonight. tomorrow the car is going into the shop near me to test for vacuum leaks. any more info is appreciated. thanks.

thanks richard all this makes sense i'm just really bummed because i have to wait for the scanner to get here and now i'm a little hesitant to drive the car lol. the guy that had it before me had no idea what he was doing from what i can see so now i'm just trying to get everything back to stock so it runs smooth so i can start doing some body work. thanks guys for all the help so far. and what are/do the two KOEO and KOER that you are talking about? need it in lamen terms right now so i understand clearly. thanks again. i'll check back tomorrow for any new info.