fuel??

5.0 Mustangs use a shunt regulator that returns excess fuel back to the fuel tank. If the return line gets kinked or partially clogged, the pressure will go up more that it should.

For this reason inspect the return fuel line before buying a adjustable regulator. With stock fuel systems, the larger of the two fuel lines is the return line. Look for sharp bends, flat and dented places in the return fuel line and flexible connectors.

Check fuel pressure:
The local auto parts store may rent or loan a fuel pressure test gauge if you don't have one.
Disconnect the vacuum line from the fuel pressure regulator. Check it for evidence of fuel present in the line by removing it and blowing air through it. If you find fuel, the fuel pressure regulator has failed. Reinstall the line, cap the open end and stow it out of the way.
Connect fuel pressure test gauge to Schrader port located just behind the alternator.
Turn the ignition switch on & start the engine. Observe the pressure : you should see 37-41 PSI at idle.
Turn the ignition off, reconnect the vacuum line and disconnect the fuel pressure test gauge. Watch out for squirting gas when you do this.

You have two choices:
Troubleshoot, find and fix problems.
Throw parts at it until you either run out of money or get lucky.

The first approach is cheaper and more productive. The second method is expensive and is a pain in the wallet. Always fix what you know is wrong first, because sooner or later you are going to have to fix it anyway.

The 24 lb Cobra MAF is a mismatch for the computer's internal program. It seems to ban-aid and cover over some of the basic problems you have.

The MAF body and sensor are designed to match each other. Therefore you can't swap sensors between different part number MAF bodies and maintain proper calibration. The assembly is designed to match the computer’s internal program, and swapping a different MAF can upset the computer's calibration. In other words, yourCobra MAF won't match the calibration of your 5.0 Mustang computer.

The only other MAF that is a one for one swap for a 93 and earlier 5.0 Mustang is the 94-95 Mustang MAF.

94-95 Mustang GT MAF - $40-$100. It is 70 MM instead of the stock 55 MM on regular stangs built prior to 94. It uses a slip on duct on the side that goes to the throttle body and a 4 bolt flange on the other. You need a flange adapter to fit the stock slip on air ducting that goes to the air box. Wiring plugs right in with no changes. *1 *2

Once your replacement 70MM MAF is in place, disconnect the battery for about 10 minutes. When you reconnect the battery and start the engine, the computer will relearn the settings for the new MAF.

*1.) Metal flange adapter http://www.kustomz.com/cat3.html Buy the TR70 for $44.95. Or spend some time on eBay looking for one that may fit.

*2.) MAF & sensor interchange
The 94-95 Mustang 5.0 MAF & sensor is also found on:
1995-94 Mustang 3.8L F2VF-12B579-A2A,
1994-92 Crown Victoria 4.6L F2VF-12B579-A2A,
1995-94 Mustang, Mustang Cobra 5.0L F2VF-12B579-A2A,
1994-92 Town Car 4.6L F2VF-12B579-A2A,
1994-92 Grand Marquis 4.6L F2VF-12B579-A2A,
Evidently the –A1A, -A2A, AA, etc. on the end of the part number is a minor variant that did not change the operating specs. You should be able to ignore it and have everything work good.

+1. Treat the problem, not the symptoms. The clear problem is that your fuel pressure is running much higher than it should.

What was the fuel pressure like before you replaced the regulator? While a fuel pressure regulator could be the culprit even if it's new, chances are that if there was no change, then there was nothing wrong with your old regulator and your problem lies somewhere in the return line. Inspect for kinks etc... in the return line first. Likely, you'll find the issue. If not, then the line is probably clogged.

Once you've repaired that problem, if the symptom still persists, then there's another problem out there.

BTW, low fuel pressure with key on/engine off, is another issue, but if you're able to maintain 65 psi at idle, it's not a serious one.