Smart proxy clips fine, together they drop frames

[ianincrete]

I'm having problems with what I assume are dropped frames. I made three smart proxies of panning shots and put them on the time line. When I chose "clip" and viewed each clip separately they played back OK. But when I chose" project" and watched the three clips together, all three clips dropped frames and jerked throughout the pan. I repeated the process with another three panning clips and got the same result. Can anyone suggest why this might be happening?