Car Update

So everyone told me that I should change the air filter and that should fix my problem. Dad and I changed it and it didn't do a thing >: ( So I've spent $40 (which doesn't sound like much but that should have gone towards food and gas) for nothing. I took it in to a place to get a diagnoisis for it and they couldn't find anything. No one can figure this stupid car out. Our registration is due this month. $140 which sucks but it's not TOO bad. I need to get a saftey on it this year and I was going to do that a few days after we did the spark plugs but kept forgetting about it. The other day I wrote it on my hand so I wouldn't forget and everything and I was going to run home and change and then get it done. When I got home I noticed this giant huge crack in my windsheild. Out of no where. We have had small tiny rock chips in our window for almost 3 years (we have 4 of them) and I figured they haven't cracked yet they probably won't do it anytime soon. Of course it has to happen when my car is breaking down and we don't have extra money and when the money we do have has to go to bills and stuff. Sucks. This always happens to us. Sorry I am just stressing over money and this huge doctor bill. This just sucks pretty bad.