Everyone is different as far as reaction to deca goes, but dosing and time on seem to be the culprit often. I never had problems running it at lower doses (like 250mg for 8 weeks the first time), although sadly enough at 400mg for 10 weeks my sex drive was slaughtered soon after.
In my experience, the keys to avoiding "deca dick" in most cases are:
1) Run moderate doses and ALWAYS stack with Test, in equal or great amounts (i.e. Enan 500mg/Deca 400mg)
2) Always end deca at least two weeks before the end of test (i.e. finish Deca week 10, continue Sust to week 12 or beyond)
3) Always have Proviron and/or Cialis on hand. In more serious cases of ED or dead libido, or onset of gyno, use Dostinex E4D @ 0.5mg.