car cranks but won't start!

[cam3]

My 90 300zx n/a has been giving me problems for the past couple weeks. It will start 90% of the time the first time I start it everyday but after driving and parking it it will crank but won't actually start. It does this completely at random. Sometimes I'll drive it and turn it off and it will start when I go to leave again other times it will just sit and crank. I've checked the fuel pump and it's working fine. I just put all new spark plugs in it. I've tried spraying starter fluid in the intake and starting it to no prevail. Pretty sure the only thing left is it isn't getting spark or the injectors aren't kicking on right? I believe it has an aftermarket alarm and I've read that they mess with stuff but the security light isn't on so it shouldn't be tripping the alarm system? I've also swapped around the relays under the hood with one's that are the same and they all seem to be working fine.

Any help would be appreciated if anyone else has experienced a similar problem!

[2+2whitez32]

not sure if this will help. but I had an issue much similar to this. my issue involved a crappy connection to the cas. the car would cut off and not start back up. would just turn over and over until I pop the hood and wiggle the plug some. it would finally make the connection and start. could also be your ptu. I thing it handles signal that goes to the coils. if your spraying ether into the intake and still not having any ignition then you deff have a issue with signal going to your coils. either ptu, cas, wiring, or ecu as a last resort. if you can id try cleaning connections on your plugs as well as trying another ptu. Those should work as long as the alarm isn't doing anything stupid to cause the car not to start.