I'm currently running the latest SW on the 810 and mine is fine. If you haven't already, go to the Garmin website & register your device. Next download the web updater, with your 810 connected to the computer run the web updater and verify that you have the latest SW installed.
The slope value which the 810 is displaying should be there or thereabouts, in relation to what is written on the card which comes with your SRM. The slope is a programmable value, in as much as if you calibrate the SRM and the value obtained differs from what is stated on the card/810, you can input a new value on the 810 which will be pushed to the SRM and permanently stored.
How to manually calibrate your SRMhttp://forum.slowtwitch.com/gforum.cgi?post=2903507
Updating the SRM slope valuehttp://forum.slowtwitch.com/cgi-bin/gfo ... st=4551194