1996 Nissan Maxima won't start. Mechanic can't figure it o

My car suddenly started to develop problems starting; the engine cranks, but takes forever to finally catch. If at all. Doesn't seem to matter if its hot or cold. Once it finally does start, it seems to roar-- and once running, it runs fantastic. Until its time to try and start it again! I replaced the clutch about a week before this problem started happening, and my regular mechanic (who has never worked on a maxima before he started working on this one) has done all the traditional attempts at fixing. New battery, fuel pump, starter. Nothing works and he can't figure it out. Any ideas?