'95 F350 7.5L Cranks but won't start

I have had a problem with this for a little while now. It almost always starts right up but sometimes, it will just turn over and not start, then after waiting for a short period it fires right up. Runs great when it is running, starts right up like a champ any other time. Now it won't start at all! Checked fuel pressure, checked spark, ran diagnostics and everything checks out. Any ideas?

Did you check fuel pressure with a gauge?

No I did not, I only checked for fuel at the shraeder valve on the fuel rail.
Now my mechanic tells me that there is no spark.
Okay, the ignition coil was replaced and that did the trick. FYI