Just a guess, but does it improve matters if you declare @tempa as my inside the loop? eg.

If @tempa is not local to the loop it doesn't go out of scope and so your references to it may be being overwritten each time through the loop.

(Added note: Even if this doesn't fix the problem, it's still good practice to limit the scope of such variables. I'd also suggest calling it something slightly more meaningful like @yvals perhaps.)