The fuel pressure check sounds like really good advice to begin with based on your initial info. If it's starting / running with starter-fluid, then it seems like the ignition system is functional - good news because I think these old ignition systems are more difficult to diagnose. As
dcm0123 mentioned, do you hear the pump prime at key-on? Can you check fuel pressure at the fuel rail? If you don't have experience doing this sort of test, it's not difficult, but please be very careful because you're dealing with fuel under pressure and there's a lot of potential for fuel leaks / spray. It sounds like you may have an intermittently failing fuel pump, failing pressure regulator, clogged filter, or an electrical issue related to the fuel pump (relay, wiring...). A fuel filter swap may be a quick first step if you're ready throw a cheap part at the problem, especially if it hasn't been changed for a long time.
Keep an eye on your battery voltage while running these tests. Repeated / extended cranking can suck down the battery quickly and it becomes extremely frustrating to deal with a dead battery in addition to the original problem. You may have to pause and charge the battery after a couple rounds of tests. I wouldn't recommend keeping a battery charger hooked up while checking fuel pressure or during any other fuel system test that can result in a fuel leak.
It might help us to know your general experience level with automotive work. If you're using starter fluid, which is fairly dangerous on it's own, then you've probably got some good foundation in engine work to begin with.
Good luck! I hope it's something simple to diagnose and fix.