IPhone 4s keeps cutting out.

I've seen a couple of these around and while they are helpful none are really answering the question I have!

A few days ago at work my phone cut out in my poket but I didn't think anything of it as it came back on when I pressed the top button, however today I was in the car with it plugged into my sony stereo via USB. Now I have been doing this for 2 years (both with this phone for 1/2 years and my 3g before that) At this point it cut itself out and wouldn't come on, so I left it unplugged from my car and went to do my shopping, when I got back in the car I tested it to see what would happen and it continuously went onto the apple screen then cut, apple screen cut etc. so i unplugged it for most my journey with the intention of plugging into my laptop to restore it.

No such luck I plugged it into my laptop and it did the same as my car, so my dad got his old cable and we poped it into the mains, Hurrah it came on! showing 100% power I thought unplugging it would be fine so I did, cut out the second the cable moved. Great. So I plugged it in again and unlocked it. By doing this I noticed that I wouldn't pick up any signal and then as it's going into 'off' mode the screen freezes at mid darkness and wont budge. I have tried to do a hard reset on it a few times (As per all the other instructions) and it has now been left alone and hour and a half -2 hours and it's still not doing anyhting differently.

Any help would be aprriciated and I really don't want to have to buy a new phone as my car needs fixing too!

Everything on the phone is up to date and I plug it into my computer once a week to back it up. The laptop I am using is a 3 year old white macbook.

Thank you in advance!