speedo cable

I have a 93 Legacy type L 5 speed manual, and i am now on my second speedo cable since i bought the car a year and a half ago. The car only has a 108,000 + - miles on it. is this a common problem, and if it is not, is there a way to make sure i dont have to replace it again? also, i have just replaced the clutch, i am wondering if i tightened it too much after putting the tranny/ engine back together. PLEASE HELP!!!

I am currently tracking speed by my RPMs not very effective.

also with this problem i have noticed that my speedometer shows that i am going five miles per hour faster than what i really am going.