is there a "fix" to prevent tranny oil from coming up and thru the speedo cable,...I have replaced the cable from another car, but this cable seems to leak even worse and leaves a small amount on the floor mat

I'm assuming you've got an EA81 based car? (seems to be the most common with speedo troubles.) You'll need to replace not the cable but the seal that goes around the cable where it exits the tranny. This seal dry-rots and tranny fluid ends up spiraling up the cable, into the speedo or somewhere else on its way to the dashboard. It's almost like a corkscrew, or an oil well drill, in the way it spirals and spews oil everywhere once the seal gives way.