O2 Sensors keep triggering SES Light, even after Replacement

Background Info:
I have a 2000 Venture, 3.4L, non-AWD. Soon after I bought it last year, the SES light came on. I had the codes read at Advance, replaced the rear O2 sensor, and cleared the codes. About a week later, the SES light came on. I had the codes read at Advance, replaced the front sensor (PITA), and cleared the codes.

The Situation:
The SES light keeps coming on. About once a month I have the codes checked, and both O2 sensors indicate "Voltage lower than expected". So I just have Advance clear the codes and I drive away, knowing that both sensors are new. I asked them at Advance, "What else could trigger the O2 codes?" but they don't know.

When replacing the Sensors, I looked at the mating connectors and saw no corrosion. I also looked at the wires, and saw no evidence of brittle-ness, rubbing, etc.

If they are working correctly, low voltage would translate into the motor running too rich. Maybe the problem is there?

Please help!

A fuel pressure test (both running and standing) might be revealing. Rapidly falling pressure after shutdown can indicate a leaking injector or FPR.

Many things can give a 02 sensor code.
As said fuel pressure.
Also maf sensor engine miss lean or rich running engine and wiring and fuses.

What was the code no.?

I don't have the code numbers recorded, sorry!

I thing I did notice, is that the light "lights" when the engine is under load. In other words, normal driving or highway driving does not turn on the light. But if we are climbing up a steep grade, *bing!* the SES light turns on.
:(