Car turns over but doesn't, start

Car will turn over but won, t start sometimes you are lucky to catch it and it will start most times it won, t. Have had injectors checked they are ok injecter seals ok will start with bumb Down hill have been told things like fuel pressure regulator and fuel rail pressure but no one can fix going to try myself can you help by giving me suggestions or tips on how to check different possible problems all mechanics that have tried to sort just want to waste money replacing every possible part. Please help if you can.