Kolchak: The Night Stalker. It was on ABC in 1974-75. It's available on DVD now, and sometimes the SciFi Channel will show it.
Many of the episodes were written by Richard Matheson (who wrote some of the most famous episodes of The Twilight Zone and Star Trek and also wrote the book which began modern Zombie fiction, I Am Legend) and all star the epically talented Darren McGavin.
The series itself has far-reaching influence, it being the inspiration for The X-Files and the book series/TV series The Dresden Files.
Unfortunately, said influence also caused ABC to try and remake the series, which ended in epic failure.