For a little while now i've had the problem of my speedometer needle randomly jumping around when I wasnt in motion. Then engine didnt rev or anything, the needle just jumped around. A couple of weeks ago, the speedometer finally stopped working all together, so the odometer isnt rolling over. Now when I try to drive it, the transmission is lagging and is having a hard time changing gears like normal. The problem seems to be better, although still not normal, when the overdrive is off, but when overdrive is on, it wont switch gears at all. What would most likely be causing this, and is this something that can be tested/repaired myself?