This happened to me a dozen times already in the last 4 years. I was never able to catch the moment it gets reset but sometimes it shows som single digit number after I just drove 25 miles to the office or back home. I though it only does that in a pouring rain but it did it once when it was nice and dry outside.
No CEL's, no other issues, stock ECU.

 

This may sound dumb, but are you sure it didn't go from 999.9 to 0?

 

That would be funny if that was the issue.

 

Probably a rod bolt.

 

+1

 

is the blinker fluid full?

 

Check to see if the Reset button is stuck.. or nudged in there just enough that with vibration it actually pushes the button.. Try resetting it yourself to verify that it works. The only other means of figuring it out is that the powersupply to that particular unit is failing on you and resetting similar to how it is when you disconnect the battery. It's a long shot but that's all i can think of.

 

is this true? I refuse to beleive it can't handle more than a thousand miles

 

reset works just fine.

I'm the only owner so I rule out anyone messing up with the electricals.
so could probably be a bad contact or a fuse that gets loose. I checked the fuses they seem to be pretty secure. Just thought it could be something common like some connector getting wet or something like that since I mostly experienced this driving in a pouring rain.

 

The trip odometer only goes to 1000 - just like 99.9% of the other cars on the road. The overall odometer will read up to 999,999 miles. Now if the overall one gets reset, we would all like to know how you did it.

 

Have your chip checked,which on that model should be behind you clocks,they do tend to go weird after time,and it just should need replacing ?

 

hmm, in the last few years I used trip odometer in es300, 2.5rs, s2000 and various e36 to see if it's time for an oil change at 3.5k miles. none of them rolled over at a 1000 miles