pedals wearing out?

Hey all -
Due to ongoing knee problems (and subsequent million things tried over 7 yrs, including surgery: plica discovered & snipped, still no resolution) I'm wondering about something else...
I've tried other pedals (Speedplays for a while .. didn't like'em, knee pain didn't change anyway.. maybe I didn't use them for long enough though) ---

but I'm specifically wondering about something re: my Shimano 535's.

They were second-hand (but recently overhauled by previous owner) when I got 'em & the knee probs started 2 1/2 - 3 months into using them, in the flat-flat-flat environment of Toronto no less.

Prior to this I'd been working (cycle msngr) in Vancouver B.C. for a year (read:hills, 8 hrs per day on the bike) w/o ever experiencing any knee pain - albeit on flat pedals...Overall I'd been riding seriously for a year before that.

(Basically I'm saying: it wasn't a case of too much mileage too soon)

I can't remember how the pedals first felt - but right now, I've noticed that my foot feels unstable on them:
Unstable in the sense of rocking (pronating/back and forth) - like the action that a canoe makes in the water when you step into it.

Do new Shimano pedals NOT have this happening?
Obviously on a FLAT pedal, one's foot would NOT be rocking upon the surface.
BTW I'm not talking about "float" - plenty of that. I'm talking about the feeling on the SURFACE: the bottom of your foot bed rocking over the pedal bulls-eye.
Best description: The way a canoe rocks back and forth in the water as you step in.

Thoughts?
:confused:

If you're feeling your foot rocking back and forth then you may need to put new cleats on your shoes. I replace my cleats every year.

If the feeling is coming not from your shoe (that is, the shoe/pedal connection) but from the pedal, then you may need to service the pedals with new bearings and grease.

Thanks for your prompt reply.
I can't tell if it's pedal interface or pedal itself (I think it might be both actually) .. will give you suggestions a try.
Thanks again. :)

Hey all -
I just wanted to inform you that:

New pedals seem to have solved the problem! ...
Upon further investigation, I discovered that if I took the Right pedal in my hand (right knee was worst) I noticed that it actually wobbled from the pedal spindle! Like, REALLY wobbled.
Well.

I got brand new Shimano 540's in August sometime after my first post regarding this.
And guess what? I'm riding more than I've been able to in .. uh, forever!

I attribute this to this unknown problem - no matter what I was having checked out (by docs, by LBS, by whoever specialists in all things bicycles) and no one had ever thought to check the play in the pedal/pedal spindle!

I am so thankful and wanna share this info, just in case anyone ever experiences this - it's one more thing to check. I believe this was always the root of the problem, as the pedals had been bought second-hand after the previous owner had just overhauled them....hmmm?

Anyway, thanks for your thoughts and Happy Riding to All.
I'm Back! :)