At Server Health, on the Average Hardware Usage card, do you consistently see negative percentages? Does that negative value appear on the actual report for Average Hardware Usage as well? If it does, one thing you can check is to ensure the actual counters within PerfMon for CPU\Memory are reporting the proper values. If they are messed up, you might need to rebuild your performance counters.
If this happens inconsistently or different values are seen on the report vs. card, we might need a support ticket to review this issue one on one. Unfortunately I'm not able to replicate this behavior.