On one leg or two legs?

My guess, for what it's worth is that you have multiple ground rods. The two at the main panel are correct...but that one separate, even if connected to the main grounds may be the culprit.

Ghstudio
I tried to disconnect it, no effect.

1. When everybody says "isolated ground" does it mean ground bar in the panel isolated from the metal enclosure?

I don't know about everyone else.... But that meets the definition as far as NEC. Then from the isolated ground bar an insulated equipment ground wire would run back to the main panel and connect there.

And, if it's not isolated from the panel (like in my case, it seems), would that be a problem causing ground loop?

No.

2. You'd have to forgive me, but I'm not entirely clear about the last part of your post;
"More than likely here is your problem. Just bet you have a difference of potential, voltage, between the equipment ground at the receptacles and the neutral, the grounded conductor.

I was going by your statement:

...... is connected to a ground bar in the panel, and the ground bar is connected to a copper ground rod right next to the panel, and to a common house ground elsewere.

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Sub panel is connected to the main one with 4 AWG wire, with Black, White, Red and naked copper ground conductors.

Well the #4 awg copper wire is good.... Actually good for 70 amps....

Not sure what all the bare, you call naked, copper ground wire is all about.

I assume the insulated #4 awg wire is installed in a conduit back to the main panel. Are the bare ground wires in the conduit? I have a feeling the answer is no.

Per NEC the feeder equipment grounding conductor shall be installed in the same cable, or raceway, as the feeder current carrying conductors. And the equipment grounding conductor shall terminate in the same panel the feeder is fed from".
And how exactly should I correct this problem?

Call the electrician and have him redo the job per NEC at no charge to you.
>>>>>>>>>>>>>>>>>>>>

How big is the conduit for the sub panel feed? 1", or
1 1/4"?

Approximately how long is the run?

Because of the 60 amp breaker used that feeds the sub panel the equipment grounding conductor would only have to be a #10 awg copper wire. (NEC 2005 Table 250.122)

If it were me I would have the electrician install an insulated #6 copper wire. Minimum, a #8......
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Ozzy asked the question, are the branch circuits hots connected to the same leg in the panel?

Post from a member on AA

Also check the receptacles for the proper AC polarity.

I made pictures of the both main and sub panels. You can see it in my system or try the link below.

http://cgim.audiogon.com/i/vs/i/f/1233285302.jpg

http://cgim.audiogon.com/i/vs/i/f/1233285348.jpg

Jea48,
As I posted above, there a pictures of the panel, that should answer some of your questions.
4 AWG wire is not in a conduit, it's the thick white one in the picture, and the bare copper ground is inside of the white insulation, with the black, white and red ones.
The run for the 4 awg is just about 3-4 ft.
3 of the hots appear to be connected to the L leg, and the other two to the R leg (see pictures).
Polarity in the receptacles is fine.
What do you think about ground from the cable connector box?, it's the green insulated wire going into the main panel from the left. (see pictures).