Another Lambda question

On a 92 300E, I was checking the duty cycle with a Fluke 88. After getting the engine warmed to normal operating temperature,I increase rpm to 2500 for two minutes. When switching to duty cycle it would run 49-51%for 30 seconds then would steadily drop down to 11.9 and hold. I then adjusted it back up to where it would fluctuate 48-52%.This caused a hard start with a rough idle for the first few seconds. Should the duty cycle remain within the 50% range indefinately or is it normal to drop this much? What could be the cause for the drop if it is not normal? Also I have 6.4 volts at pin 3 in the diagnostic socket, KOEO, which I believe is a bit high.

thanks in advance!