1993 Chev truck electrical problem

The truck is a 1993 GMC K1500 Extended cab 4x4 with a 350 engine in it. It currently has 198K miles and the new long block was installed in Jan/Feb of 2003. When driving the truck at any speeds the speedometer will bounce (quickly) from your driving speed to some speed 10-20mph less and then quickly back to your driving speed. This happens very frequently. When this happens no other gauges bounce or act abnormal. It doesn't appear to effect the motor, drivetrain or any electrical devices other then the Speedometer. Once in a while when the speedo bounces, all gauges bounce, the fasten seat belt light comes on, a buzzer in the dash goes off, the radio and other components shut down and the truck falls on its face. This is a very brief thing, like it happens for 1-2 seconds, then the truck recovers and all the gauges, drivetrain and electrical components return to normal. This started happening shortly after we replaced the motor and it started out with just the speedometer bouncing. Since then, it has gotten worse and now on occasion, the whole electrical system seems to shut down. After swapping the motor, I replaced one ground strap from the motor/trans to the firewall. I have also replaced the alternator. I check the computer codes and I'm getting the following error codes; (12, 42, 59, 66, 67, 73. Any help would be greatly appreciated

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My impala is a 68 and it dose that its a loose conection on the back of the speedometer try that

check the main motor harness...if its loose then it will do that exact thing...

Since you just replaced the engine, i would double check the ground wires that go to the engine. There are a couple at the back of the right cylinder head (right side of vehicle when viewed from behind) and i think there is a big one on the lower right front of the engine. Make sure none of them are rotten or coming apart and that the are connected tightly to clean, bare metal on the engine.