If you have fitted the new cable with to small a radius somewhere along its lenth then it will jump like you are describing, the inner cable catched on the outer causing the inner to get wound to a point that it overcomes the friction between them then the lenth on the speedo end catches up causing the speedo to jump, its worse at low speeds.
If you dont fix it then you will get another chance to re route it shortly when it pops.
Or just use your satnav for MPH.
Drew.