smear in PCR

[Peggy De Clercq]

I want to amplify a 224 bp product from genomic DNA, isolated from blood. 
At the beginning of my experiments it worked well but after some PCRs I 
got a smear, also in the negative. Sometimes I have samples that work 
well but in the next PCR they don't work anymore. I already bought new 
primers, changed the MgCl2 concentration, added DMSO, changed the method 
of DNA isolation but without results.
Could anybody help me please?

Peggy De Clercq