DNA from old frozen and partly coagulated heparin blood

Hi,
my standard method for extraction does not yield any DNA from this
frozen heparin blood. I also tried the Quiagen QuiAmp Midi Kit but with
no result.

Counting the WBC yielded approx. 600/ul, probably due to freezing?
Qualitative observation with Giemsa coloring, showd no intact nuclei of
WBC beside the counted cells.

Probably the problem could be that the probes are partly coagulated due
to insufficient mixing of the li-heparin blood container?

Any suggestions are welcome, the problem pertains around 200 probes,
with clinical data availiable. So it's very important for me to find a
way to extract the DNA.

Yours, Andy