Speedometer question (not a newb one) (I think)

[Clawhammer]

Hey guys,

I have a '90 fastback w/automagic transmission. Well the speedometer has worked intermittently since the day I got it. Probably about 40% of the time I drive it's actually working. Can't really pinpoint what makes it work cause it's so random...sometimes I hit a bump and it goes, others it doesn't. Sometimes while flooring it and the tach get's high it'll work but other times it doesn't. Sometimes I'm just cruizing and it'll (the needle) pop up and work for like 5 minutes or all day. Anyway, really random. I know these cars are notorious for speedometer problems and that this is nothing new, but I replaced the speedometer with one from a DOHC and it still does the same thing. So I got a new transmission speed sensor, no deal. The only thing I can think of is the ECU?? Seems like the signal is getting lost between it and the speedo. I know everything's plugged in for sure, and all the connections look fine, so I'm stumped.

Just wondering if you guys could give me any insight on the problem, I'd really appreciate it.