Crank but will not start?

4 weeks ago the emissions canister was replaced I passed DEQ ran great for 2 weeks one day it wouldn't start then the flowing day it did drove it then it did the same thing waited then 2 hours later it started got it home an there it has stat for over 4 weeks I have replaced the fuel filter, O2 sensor air intake hose, alternator, oil pressure sensors and spark plugs. Help please thank you Jolene