dlink di-604 wall-wart failed twice
hp 4470c scanner wall-wart failed once
ovislink dsl modem wall-wart failed three times (using hp 4470c ac adapter temporarily)
hp laptop power supply replaced due to loose connector

ATX power supply replaced twice

Medical rated adapters cost twice as much, but are more reliable

Bill_Jaffa coment:

Good point--it's not just the wall wart which is unreliable, but the flimsy connector 
pair as well: minimal contact area (especially the center pin, as you note), no locking 
retainer or positive "click", subject to axial and radial forces via the connector and 
cable which degrades the contact situation

recommended wall warts include:
Triad Magnetics, CUI, or Phihong

save your old DC plug and splice it onto the adapter output

probable cause of failure is bad caps

check:
https://en.wikipedia.org/wiki/EIAJ_connector

for dlink:
male EIAJ connector