[84LX89GT]

It does it sound like it's running lean. The first thing i would do is to go buy a camping propane bottle and fitting (turn on and off to vent bottle) OR get some highly evaporative brake cleaner spray (works good) and start by spraying on and around the engine to see if there is a massive vacuum leak.
Also, what is your base timing set at, if it's too far advanced the engine will backfire, lack power and not be able to ignite the air/fuel because there's not enough cylinder pressure and also if it manages to run it will heat up the cylinder/head enough to cause an induction backfire.
Does the engine run rough at all? Is there anything else you noticed. Does it do this all the time, more when it's hot, etc.
88 GT's relied on a MAP sensor (manifold absolute pressure) which is the head sensor for computing timing/fuel and calculates Barometric Pressure (altitude, weather factors, density of air) and a MAP sensor can go bad and the processor will be none the wiser because there is no way for the processor to check to see if it's good or bad unless it has completely failed electrically. A bad MAP sensor can also cause what you're describing.
To test a map sensor you can take a small paper clip and insert it into the back of the harness side connector on the MAP sensor. There are 3 wires to the MAP sensor, one is a 5 volt referance voltage, one is a ground the other is the signal back to the PCM. So one wire will have 5 volts (roughly), one will have 0 volts (5 volts if you put Voltmeter up to battery power) and the signal back should read the following for certain altitudes:
0 (ft altitude) 1.59 (volts)
1000 1.56
2000 1.53
3000 1.50
4000 1.47
5000 1.44
6000 1.41
7000 1.39
These readings can also be taken as frequency (Hertz - Hz) and near sea level would be about 159 Hz.
You can also take your MAP sensor into a ford dealership and they can use a machine called a SBDS to see the Frequency output compared to the machine's internal MAP sensor. But for the price of doing that you could probably buy a new one anyway so that might not be practical.
Good luck,