Also be aware that contamination may not only come from reagents, but also from your work bench, pipettes etc. You should clean up those things and assemble your PCR in a designed area just for PCR. Using aerosol resistant tips is also important to get rid of contamination.
I made fresh aliquots from original stocks so I guess my stock could be contaminated, but then why no band in my untransfected cell? I keep an area designated for PCR and wipe it down, wipe pippettes down, use special tips etc. And why does it run exactly where my other bands do?