B2.66a, Win7pro both 64b.
Added first marker, tracked it.
Added second marker, started to track it, keeps disabling itself between frame 80-90.
I chose a perfectly good tracking position, high contrast, on screen the whole footage.
I can’t find any debug messages or anything else. Why is it doing that? 
We don’t see the footage, so it’s impossible to judge, but there may be several reasons, two of them being:
One: Perfectly good tracking position is not so perfectly good, but as I said - without your footage it’s hard to say.
Second - maybe your perfect tracking point is moving fast and you have too small search area.
Three - some other reason
Four - yet another reason
Five - some reason not mentioned above
Well, neither of those really.
And trust me, as I’ve written a matchmover myself already I know how to choose features
Unfortunately I can’t show the footage, was tough enough to get the material out of the client literally past an army of lawyers -.-
It would at least be good practice if Blender would tell you ANY reason why it disabled the tracking point and refuses to follow it.
Is this the only feature you have problems with? If you try to set next feature will it as well keep disabling?
Is it possible for you to upload the blend file without footage of course?
I could then simply go through the settings of your track points and maybe there is something you enabled or disabled by mistake.
Interestingly enough most features work flawelessly, just some, for no (to me) apparent reason stop at random frames. So far I had 3 features refusing to work.
Only pattern I see is that they are close to the border, but the integral image area neither touches, nor leaves the frame.
I am somewhat able to solve the camera movement, I was just wondering what the reason could be for a point to stop being tracked, simply disabling itself without any form of feedback
I’ll play a bit more. Haven’t done a lot with Blenders tracker.
You can use Blender for years, feel you know it and discover something you’re a noob in any day
Mostly used, don’t remember the name of the tool… some other guys in the last studio I worked in did all the tracking horrible work 
I suspect that it’s just getting too close to the border…In the tracking settings, do you have a margin set? But even without a margin set, I have found that sometimes the tracker acts like there is a margin of about 10 anyway. Not sure if it is a bug or not but I just try to avoid markers near the edges and grit my teeth and manually keyframe any markers that get too close until they either leave the frame or leave the border area…
Margin’s set to zero. But I share your feeling of this invisible margin
Interestingly enough, you can re-enable the marker, track a frame, re-enable the marker, track a frame, re-enable… yawn
Have you tried tracking it backwards? Maybe if they meet in the middle you can ‘join’ them.
No, but I could try that. Some time.
To be blunt, currently I don’t have the time to play around, I need to get the job done.
Still. It’s odd. You don’t terminate an operation without feedback.