2001 Kia Optima Dead Gauges

Just had a used engine put in. Purrs like a kitten, but my Tach, Temp Gauge, Speedometer do not work? I should say every once in a while they start fluctuating. Strange, they have always worked perfectly up until the engine was replaced. Strangely, I reset the odometer and the gauges are somewhat working again, when they want to. What is wrong, my mechanic seems to think it is a common ground wire or a wire that goes to the transmission, if not he says he will have to take the dashboard off and see if there is a loose connection. He said it is nothing he did, well it has to be something since they all worked prior to the replacement being done.