Lights flicker and gauge jumps around???

When I start the car sometimes, the voltmeter will read about 12V. If I rev the engine it will jump up to 14V at idle and stay. Took the alternator to the local parts store to have it tested
Two stores tested it good,one said it didn't put out the proper voltage but the diodes tested good. Watched a video which stated that the alternator should be grounded to the voltage regulator if it's external.