I had the same mystery. I don't know why starter fluid would sort of work and then not. But here is what I did find:
BAD IGNITION COIL CONNECTION TO WIRING HARNESS
It seems that the connector gets old and brittle after time. Becomes an intermittent problem. It is probably common and why there are so many posts about this. It was a devil to figure out.
I had a similar issue on my 1998 Hyundai GL (stick shift). After replacing the battery, fuel pump and relay, fuel filter, checking all the sensors, checking out the ECU which gave no codes, and paying a lot of different mechanics who couldn't figure it out I decided to try replacing the ignition coil. After that is seemed to work, then went a little funky. I discovered that it was running on just 2 cylinders. When I wiggled the wires to the ignition coil it resolved the problem which then came back. Upon swapping back the original ignition coil to test I found that the connector from the wiring harness was very brittle and after fiddling with it I realized that this was a problem. Not able to find a replacement connector easily I drove it around on 2 cylinders with one of the leads hard wired. It then consistently ran - but on only 2 cylinders. I then hard wired the other 2 yesterday. That seems to have fixed the issue. It was as simple as that. Intermittently bad connection to the ignition coil.
If you need to replace that connector it is "Ignition Coil Wire Harness For 96-00 Elantra, 97-01 Tiburon 27367 23000" Beware that Hyundai uses the same part number for the condenser part - so verify that. I had to wrong part sent to me twice before I gave up and hardwired it. That part costs about $30 if you can find it. Thought I would share.
Monday, July 24th, 2017 AT 3:06 PM