I had the opposite experience. I used to have knee problems riding in clips. I switched to clipless mainly because they looked silly on my road bike (such vanity!) and my knee problems cleared up soo after. I think that was because riding clipless pedals encouraged me to adopt a straighter leg position whereas in clips my legs could flop and move around. Having a secure foothold was also better. Every tried riding in clips with the strap a little too tight! Not for the faint hearted! I've never slipped my foot off the clipless by accident, and even full standing hill climbs made me more confident. Again though, it's a personal choice, but well worth trying if you've never used them. Maybe try a mates bike that has them fitted. They take a little getting used to but that soon passes.