91 Max Speedometer/Odometer

Hi,
Every morning when I start my 91 Maxima, the speedometer/odometer work for about 2 minutes, then quits when I stop at a light, slow down. Happening for 2 wks, using my tach to approximate my speed. Could it be the cable or cluster, or? I'd appreciate any advice before I bring it to a mechanic and I don't want to get ripped off. Thanks.

Maxima has electronic speedometer =no cable:
---> speed 'sensor', wiring loose or speedometer itself kaputt.

Read the forum, somebody else has had the same problem and there's info how to change. Sensor change no big deal I suppose. Instrument assy is quite fast to take off...

1. "work for about 2 minutes" ---> after warmup... quits
2. "when I stop at a light, slow down*" ---> here* engine changes slightly position (wire, connector?)
1&2 points under the hood, sensor or wire at gearbox. Maybe. Check, clean first.