I've been wearing bi-focals since I was 39, and tri-focals since I was 45. I strongly prefer lines over no-line. No-lines make me feel like I'm looking through a telescope--only a pinpoint is ever in focus.
I'm particularly sensitive to line position. When my frames are out-of-whack and my lines don't line up, I have trouble.
I also prefer my lines down further than most people. Opticians hate this. They always want to put them where they think they should be (even with the bottom of the iris) and that just drives me bonkers. I have to be *very* assertive (read: threaten to go elsewhere) in order to get them where I need them to be.
Either situation (lines uneven or lines too high) gives me exactly the symptoms you describe.
Edit: You should be able to see if your lines are level. You can also test for yourself if your lines are too high. Slide your specs down your nose. It'll feel funny, but if your vision improves, there's your answer, and have new lenses made.