Paranoid boyfriend- please help!

I'm sorry this is so long.

I've been with my bf over 4 years, but I don't know how much longer I can take his paranoia. He is EXTREMELY jealous and always convinced I'm lying about things when I'm not- especially about where I am. His 'evidence' is shaky at best, and plenty of times I have significant proof that I was where I said I was (browser histories on my home computer, receipts, etc.). He says I can never really prove anything and insists upon my 'confessing' before he will stop. He will spend hours like this. I'm ashamed to admit it, but at this point, usually I just stall for long enough to figure out a plausible scenario in which I *could* have been lying, and just 'confess'. He gets pushy, and makes a really big deal of out my "lack of respect". Most of the time during these fights I'm only allowed to speak when spoken to- I wonder who's not getting respect. Really, the issue is that I don't honestly think he's doing it to be malicious; I think he genuinely has a problem. My question is whether it could possibly get better. As it is, I just can't take it anymore.

I know that given this, most people will most likely just tell me to leave him- hell, I would, if I was the one reading this post about another person. But I know that when he's not like this, we actually have a wonderful relationship. Apart from this, he's everything I want in a boyfriend- smart, kind (except for when he gets paranoid), loving, faithful. I know that's the way with all of them- they're very charming until you make them angry- but as irrational as I know it is, it's the reason why I've stayed for so long in the first place. I remember that most of the time, things are great, so I try to minimize the fact that dealing with his suspicion is hell.

Any help from anyone who has been there, or has any insight would be very much appreciated. Don't try to make some slippery slope argument about this- I've been with him for over four years, and the situation hasn't actually gotten any worse, it's just been wearing away at me. I seriously doubt he is going to start to be abusive all of a sudden, I just want to know what to do about how he is now.