I found a few patents on this subject. I found the info below in the following patent application: (#20090021452)
While it may not describe the exact issue you are seeing it sure sounds like it. There is tons of info in these paragraphs but I’ve highlighted (bold) the most important parts IMO. If you want any of this translated into plain English(LOL) just let me know
Next, the relationship between the power-on accumulated time and initializing voltage Vr is described. FIG. 13 is a pattern diagram showing the relationship between the power-on accumulated time and discharge start voltage of panel 10 of exemplary embodiment 2 of the present invention. As the power-on accumulated time of panel 10 increases, the discharge start voltage gradually increases . The discharge characteristics vary over time, so that the following problems occur. When initializing voltage Vr is set with reference to the discharge start voltage of panel 10 having a short power-on accumulated time, extending the power-on accumulated time increases the discharge start voltage. At this time, initializing voltage Vr becomes relatively low with respect to the discharge start voltage, hence the initializing discharge becomes weak, and sufficient wall voltage cannot be generated or the priming runs short. Thus, the initializing discharge can be insufficient. While, when initializing voltage Vr is previously set to be high in expectation of the variation over time of the discharge characteristics, initializing discharge becomes stronger than necessary in panel 10 having a short power-on accumulated time, light emission that is not related to the image display becomes strong, the black luminance increases, and the contrast can degrade.
However, the panel of the present embodiment has accumulated time measuring circuit 83, measures the power-on accumulated time, and performs control so that initializing voltage Vr increases with extension in power-on accumulated time. An image with sharp contrast can be displayed without un-stabilizing the initializing discharge regardless of the power-on accumulated time from the initial stage of the manufacturing of plasma display device 1.
An example of the method of controlling initializing voltage Vr based on the power-on accumulated time is a method of continuously increasing initializing voltage Vr with extension in power-on accumulated time. In the present embodiment, a plurality of accumulated time thresholds are set, the power-on accumulated time supplied from accumulated time measuring circuit 83 is compared with the accumulated time thresholds, and initializing voltage Vr is increased whenever the power-on accumulated time exceeds the accumulated time thresholds. FIG. 14A and FIG. 14B show a control method of the initializing voltage and a transition of the contrast ratio in accordance with exemplary embodiment 2. FIG. 14A shows the relationship between the power-on accumulated time and initializing voltage Vr, and FIG. 14B shows the relationship between the power-on accumulated time and the contrast ratio. In exemplary embodiment 2, three accumulated time thresholds of 650 hours, 850 hours, and 1000 hours are set, and initializing voltage Vr is increased by 5 (V) when the power-on accumulated time exceeds 650 hours. The increase in initializing voltage is performed not only in normal-temperature driving mode but also in low-temperature driving mode and high-temperature driving mode, and regardless of the APL value. When the power-on accumulated time exceeds 850 hours, initializing voltage Vr is further increased by 5 (V). When the power-on accumulated time exceeds 1000 hours, initializing voltage Vr is further increased by 5 (V).
In the present embodiment, as the discharge start voltage is increased in response to extension in power-on accumulated time, initializing voltage Vr is increased. Therefore, image display with a contrast stabilized at a high level is allowed regardless of the power-on accumulated time.
It is considered that the panel is hardly affected by increase over time in discharge start voltage when initializing voltage Vr is high, so that increase rate of the initializing voltage may be decreased as initializing voltage Vr becomes high.