Clean your battery terminal connections, if the problem persists have your batteries checked to find which one is weak. You have a voltage problem most likely.
Mine started doing that right before the tranny went south, like u said at about 50-60 in OD it seemed like the TCC was locking/unlocking rpms would jump about 100-200 then return to normal after a second or two. Check ur trans fluid make sure its not dark or smells burnt..if all is fine with that then I'm not sure.
Basically a noise filter is a coil of wire that will help weed out small irregularities in the current, you can pick one up at Radio Shack. Now where it goes in at I couldn't tell you, there is a reason I bought a stick shift LOL LOL