Circuit with faulty ground

[dwfmcneil]

I have a 1950s house with ungrounded outlets. The wiring has a bare ground, a little smaller than the black and white insulated conductors, and it is accessible. I've replaced all the ungrounded outlets with new grounded ones and connected the ground wire both to the metal outlet box and the green screw on the outlet. For all outlets but one, my little Sperry tester says they are wired correctly. On the bad one, my voltmeter shows 120V across hot and neutral, but only 20V across hot and ground.  I can't find anything on the circuit except two outlets and a switched overhead light. The voltage tests the same at both outlets and the switch. I haven't pulled down the overhead light yet to test there. Any suggestions as to what the problem might be?
Thanks for any assistance!

[JP]

Sounds like the ground is open at some point, It needs to be a full circuit back to the panel, and it also helps if the panel is grounded correctly.