Tach/ Speedo Error

[bill88]

Has anyone experienced or know what could cause the tachometer to read both too high and too low sporadically? Mine does, sometimes reading 2500 rpm at 70mph and other times 1700 at the same speed. When it reads on the lower side it will show zero at idle. I also get speedometer errors usually involving it not returning to zero when stopped (bottoms at 5mph), seems to be accurate otherwise. Both function normally at start up and for the first 30 minutes or more, then the problem starts. I can't tell if this is a sensor problem or an electrical / voltage problem. Any thoughts? Thank you in advance