Nissan Sentra is worthless

I bought a brand new 2007 Nissan Sentra. It now has 44,000 miles on it. I have just recently gotten my car back, after having it worked on almost the entire month of July and part of August. Took it to a Nissan dealership when it first started acting up and they couldn't find the problem Took it to them a total of 5 times and paid around $130.00 for nothing. No work, nothing fixed. Finally took it to another mechanic and ended up spending around $500.00 with them only to have it tear up again. Nissan finally replaced the engine after 3 weeks of going back and forth about my engine warranty. Got the car back, and now it has started acting up again. Sometimes it will crank and sometimes it won't. It's not the battery. I have to try sometimes 15 times before it will crank. You can hear it trying to start but it won't turn over. Called the dealership and they said I could bring it but I would have to pay again for them to run diagnostics on it. And they couldn't give me something to drive until they determined what was wrong. I think they should look at it for free considering they just replaced the engine less than a month ago, without me paying them again. What options do I have??? Also, what recourse do I have with this vehicle? It has been one problem after another.