I run a TOR relay under the name "torland" (http://torstatus.blutmagie.de/router_detail.php?FP=8d90596192e548c8c00fa9e82759f95b875dc99e). Just today, my relay became marked as a stable tor relay because it has been running non-stop for around two weeks. However, my concern is: I'm going to need to shut my relay down eventually when my computer needs to do a software update, or if my internet connection is turned off for whatever reason. Is there a way I can keep my relay marked as stable when this happens? Thank you.
Restarting a relay will not cause it to lose it's Stable flag. Stable doesn't require 100% uptime, it requires that it be up most of the time, so restarting it to apply updates shouldn't cause it to lose it.
"Stable" -- A router is 'Stable' if it is active, and either its Weighted MTBF is at least the median for known active routers or its Weighted MTBF corresponds to at least 7 days. Routers are never called Stable if they are running a version of Tor known to drop circuits stupidly. (0.1.1.10-alpha through 0.1.1.16-rc are stupid this way.) To calculate weighted MTBF, compute the weighted mean of the lengths of all intervals when the router was observed to be up, weighting intervals by $\alpha^n$, where $n$ is the amount of time that has passed since the interval ended, and $\alpha$ is chosen so that measurements over approximately one month old no longer influence the weighted MTBF much.
This is by intent, since obviously relays should have some amount of downtime to do sensible things like install software updates.