real-time PCR problem - (Jul/25/2008 )

Hi,
I took over qPCR from previous person for horses. All genes worked well but B-action. From data file I can see she did B-actin optimization which everything is good by using plasmid DNA. Recently I repeated this qPCR by using plasmid DNA many times for standards. The graphics are mess and E is very low. I re-prepared plasmid DNA which is good by seq check and I use new sybe green mix from Bio-rad.( the primer was in 3/2006). The primer was aliquant. It looks like everything is right for me. Does anybody know what’s wrong? Any comments and suggestion will be welcome.

Thanks very much,