08 Mazda 3 sometimes won't start
 

I'm not a particularily mechanically inclined person, so hopefully somebody can help me out. I have an 08 Mazda 3, I believe it's a 2 L, I've got 132,000 km on it. I bought it new (Oct '07) and it has never given me any problems until this month. I haven't driven it much the last year, and it sat idle most of December (2013). I had to boost it early jan to get it going. It ran fine for about 2 weeks. Then it started to need a boost every other day, then everyday and now almost every time I need to drive it. I had the battery checked (it was 75%), and was charged from there. I was told if it cranks, it's not a problem with the starter. Because it is holding the charge, it's not the alternator. It runs after the first attempt with a boost, so it wouldn't seem to me to be spark plugs. Does anybody know what it might be, how to fix it, and what I'm looking at for a cost?

Thanks

7 years old, been sitting idle (not sure where you're located, in the cold?), and nothing else seems to be wrong.

I'll bet your battery isn't holding a charge like it did in '07.