2002 Rough start

Im really new to the whole import/newer car scene. Im used to working on older cars with basic computers and distributors...so this problem is out of my reach.

So I had a clogged cat and the car wouldnt rev above 2500rpms. Gutted the cat (temp fix) and after that the gas mileage dropped massively. So I thought that it might be fouled plugs...changed them and thats where everything went to hell...

When you try to start the car sometimes it wont turn over. It tries but wont catch. Although when it actually does catch it runs perfectly...the gas mileage improved a good bit to. So the only problem I have is the engine starting.

Any help would be great.