[GapBoyPCS]

Most cases I've heard of sweeping gauges suggest a charging system malfunction. So, you're on the right track.

Check to see if the battery cables are on the terminals tight and/or if there's any corrosion. If the shop replaced the alternator, make sure the cables/connectors are on nice and snug. Also, check electrical grounds too. Since the alternator went on you, it might be a sign your battery is dying (due to improper charging). Have your battery tested too.