GMC 500+ 'lags' behind GMC 500 by 10 CPM (.5 usv/hr).

Hi,

Any help would be appreciated. A few days ago, I purchased a GMC 500+ and have noticed that it lags behind in measuring radiation to the GMC 500 I have. By this, I mean the 500+ will 1) catch up and match the readings of the 500 after a while, 2) lag behind by around 10 CPM, 3) and rarely measure over 30 CPM. The only thing I noticed that may be an indication of something being off is in the Tube Settings, 'Tube Selection' states 'Tube 1, Tube 2, (need to reboot)'. I tried doing a factory reset and checked to see if anything looked awry with the tubes, but the issue still persists. I wonder if 1) there's an issue with the 500 + 2) what could explain the discrepancy in measurements and 3) is there an easy way to fix this?\

Unrelated to this is the GMC 500 and 500 + are between 100 - 150 euros. Are there other meters around this price range or between 100 - 250 euros you recommend?

Thanks.