Is the neutral bonded to ground, or floating? If it was permanantly installed, they may have removed the bonding jumper- good for connection to an 3-pole ATS, but bad if trying to use it as a portable. Might be a loose/dirty connection on phase B, too.
The frequency is directly proportional to the speed. If the speed isn't varying (you'd hear it for sure), then I'd suspect the meter is giving you a bad frequency reading. I'd check the neutral first, as that might be what's throwing off the meter.