From your description, the switch sounds bad. I know that you replaced it, but with what? a used part or new part?
If it was a new part, I would clean all connections going to that switch, make sure to use dielectric grease when reinstalling. If that doesn't clear it up, I honestly think that you should investigate the computer... I don't know about headlights, but when I removed my alarm system from my mustang, the blinkers didn't work. When I put it all back together, the car would start with no blinkers however. I changed the computer due to starting issues (after some time), and ther frickn' blinkers started working again. I was surprised how many things are controlled by the computer in this car.
But it really sounds like you just have a bad connection. I would firtst try looking at the connections going to the lights and the connections going to the switch.