Greasing Speedo Cable.

[THE "10" SILVERADO]

Hey guys,
Installed a new speedometer about 6 months ago (never lubed it) and now that I am driving the truck it bounces all over the place. 0-35 only bounces 5mph or so but after it goes up to 70 when I go 45. Also Sometimes it sticks (?) and makes a tremendous noise on top of the usually bouncing noise.

I already have my dash put together so I don't want to remove the cluster, I can reach the speedometer cable housing through the back so my question is. Can I just unplug the cable, put some sort of grease on it and then re-install? Do you think that would solve the problem?

Also what type of grease? WD-40, White Lithium, Bearing Grease, graphite based???

Thanks,
Michael