89 240sx speedo jumps around

[madizmo]

Ive done alot of searching on here and havent really found anything about a speedo just surging, mine works perfectly fine and seems to be reading acuratley but sometimes it randomly jumps around then goes back to normal. Today i was doing 100kmh and it jumped to 140kmh for a quick second and then returned, my car has also been hesitating between 2500rpm and 3500rpm would this have anything to do with this problem? i dont know where to start to look to try and fix this problem. any help would be much appreciated. Thanks in advance