94 4.0 intermittent starting problems

My '94 4.0L is having very intermittent starting problems. It will turn over fine but wont start for a while. Seems to happen most often when hot (after stop and go driving) but occasionally when cold. I've replaced the fuel pump relay but that didn't seem to solve it. I'm not sure if its spark or fuel that is the problem since it's so intermittent I haven't had it happen at home when I can really check it out. Any ideas out there?

If it's taking a long cranking time, but it does start AND runs normal with full power afterwards you could be losing fuel pressure while the truck is sitting off. If you can get a pressure guage, do a pressure leakdown test. An easy trick could be to cycle the key to "on" but not "start" at least three times before starting. If you did lose pressure, this should restore it.

my 4.0 wouldnt want to start either after getting hot.. and every day after that it got worse.. my starter ended up quiting all together... so i replaced it.. and it worked