93 s10 problem starting

Hi, my problem seems to be a little different. It sometimes starts and sometimes doesn't. When it doesn't, a quick shot of starting fluid starts it right up. It seems to me if the fuel pump wasn't working, the starting fluid wouldn't help. I just changed the fuel filter, and there was fuel pressure at the outlet of the filter, so it seems to get fuel. Also if it was the relay, then using starting fluid shouldn't help. The only thing i can think of is the oil pressure sending unit, because it will also occasionally die while idling or when pulling up to a stop sign. how do I tell if thats the problem? diagnostics of what sort, and can anything else cause this problem