Yes, temperature certainly has something to do with it. Air and/or accumulated water in the system expands dramatically compared to brake fluid. Since
every system has at least some of each in it, and both accumulate progressively with neglect of service, what do YOU think your chances are?? When temps rise, air and water vapor in the system expand, creating highly compressible "gas bubbles" in the pressure lines and in the master and slave cylinders, and you will have problems of the exact nature of those you describe.
Again, as I previously stated above, filling up the reservoir isn't going to "fix" anything here! By the symptoms you describe (they're as common as weeds as an result of clutch service neglect, BTW) at a bare minimum, your clutch system needs proper bleeding and flushing -- there's simply no alternative, and no way around it. Filling up the reservoir on top of contaminated fluid is a waste of good fluid. It will NOT make your problem go away -- or even mask it.
This does NOT appear to be some great horiffic problem, my friend. But it needs tending to immediately, or you could easily find yourself broken down and calling for a tow at a time and place not of your own choosing.
Unless you've got a failed (or failing) component (it's a possibility, as mentioned above), it's really very simply dealt with in less than 20 minutes if you have someone to help you bleed it.
Now this is just me, but I wouldn't risk driving your car around the block until it's properly bled and flushed. If you're unable to do it yourself, get a shop to do it for you.
Just tryin' to help, my friend.