Diet,

 I don't have a specific solution, but that should not be considered normal.  I am planting with an 8100 with the big old round Magnavox radar and it's speed and consequent populations are holding nice and steady - at least as far as I have noticed.  I have not checked the performance data page speed but my population is staying +/- about 200.  I agree with the other suggestions for switching to GPS as a speed source and making sure your seed drive sprocket configuration and number of seed disk cells is correct.  Not sure right now, but can you turn off the radar as the speed source and let it use wheel speed to see if that helps....?