I've had symptoms similar to these since last march. They're minor enough where the docs can't pinpoint the problem, but severe enough where I probably ride 20% as much as I used to. I'm getting totally out of shape and am getting depressed.
This is a tough one and sounds very frustrating. When you say "docs" , it makes a big difference if you mean an orthopedic doc who specializes in sports injuries or some other doc. A good orthopedic doc should be able to contort you in enough positions to nail down the most likely cause and rule in our out big stuff.
The really big stuff (torn ligaments) would be obvious. It sounds like a cartilege irritation or maybe a small tear. They've done studies where one group with minor irritiation was scoped for repair and one group with similar symptoms was not. A year later the surgery group did no better. So, don't leap to the "I need to have it scoped" mindset.
It would help your head to make a plan and stick to it. It'll give you focus and make you proactive in getting your knee back. And, you will get it back. If you haven't seen a good othro, you should. If you did that already, you could work on strength, flexibility and pain/inflamation control.
For strength, squats, leg press and hamstring curls do the trick. You need to balance what you can lift with how much irritation it causes. If its major pain and/or swelling, you may have to bail and do straight leg lifts (lying on the floor with ankle weights) or rubber bands stuff. Biking on a stationary bike and/or easy flat road is another thing to work on - no big hills and no out-of-the-saddle at first.
For flexibility stretching several times a day for hammies, quads and calfs will help you. If you have limited range of motion for full extension or full flexion, add it to the list of things to work on. If you can't go to full flexion (heel to butt) due to fluid in the joint, then more ice for you. At a minimum,.stretch in the shower in the morning, at lunch and in the evening. It's best if done when you've warmed up, but try not to skip it no matter what.
For pain/inflamation control take glucosamine every day. It'll take several weeks for it to kick in, but it will. Advil works for minor post-workout pain/swelling, but shouldn't become a habit. Ice has amazing healing powers and pain relief and should be your best friend when your having a bad knee day.
If what you have is some minor irritation and you stretch, strengthen, ice and pop glucosamine, I'd bet that in 8 weeks you feel much better and it will snowball in a good direction from there.
Check out Bob's ACL Board - it's not just for acl problems and there are therapists and patients who have seen it all. You could post your question there and have lots of company.