Grounding/filtering issue or multimeter is faulty?

[zape]

I have a weird situation here with plasma TV.
When I measure Vs and Va output with my cheapass harbor freight DMM it reads random values.
To my surprise I noticed that if I use GND output pin on the power supply instead of chassis then it reads correct values.
Then I measured across GND pin and the chassis and I get random readings again! but the ground is connected to the chassis! How this can be possible?
Is this something to do with bad filtering or some sort of noise happening? Or this noise is okay and it is just bad multimeter which can not filter out this noise?