Perhaps you should not try to play 5+ hours of play without at least saving the game now and then, so you can revert to a recent saved game.

There should always be a AutoSave.sav file that is within 10 turns or so, even at default settings. (and going back to the AutoSave.sav or PreviousAutoSave.sav, you will never lose 5 hours of game play)
I set mine at 5 turns, just so I don't have to go too far back.

If you post your debug.err contents, it might help to see where the problem is.

The max population of a planet is limited by its original size. Maybe the planet had been terraformed before you got it.

overheated?
it is a 2 yr old comp, but ive left comp on playing for over 10 hrs before playing other games, i have a dell xps,3.6 gig proc.,radeon X800XT vid card with 3 gig ram, there r 3 fans in this tower, nosiest comp i ever heard, maybe that all means nothing, i'll have to remeber that with next games but i have finished a few huge games with many an hour played at those also, ill have to keep track of my timen

thnx

Are you sure this is true? This hasn't been my experience. It is certainly dependant on the planets listed PQ which can be different than the number of buildable tiles, but I've never seen a planet behave differently then it's posted PQ. For example I colonize a PQ9 that later gets increased to PQ12 by terraforming, etc. I definitely know that as a PQ9 I did not get the 10% high PQ planet morale bonus but as a PQ12 it does.

While absolutely a true statement, it is perhaps a little harsh to not sympathize with the OP's issue. Also in his defense the most prevelant time for the game to crash is precisely when doing a save (autosave or quicksave) that first delete's then rewrites the original file. It's usually in the file deletion process that the crash occurs leaving you not only with no new save game but deleting your most recent save game as well. I usually deal with this by doing an explicit save to two save games that I keep current. If one messes up I at least can reload the other and then try again. Also I only play for the length of time that I'm willing to lose before doing another set of saves.

The autosave interval is 12 turns by default. In the mid part of a gigantic galaxy game I often have turns that take one to two hours (damn that autopilot bug). If I were to count on autosave and had to go back to the default interval previous autosave I could easily lose 18 to 24 hours of play. But certainly the suggestion of saving as frequently as the length of game play that your willing to lose is a sound suggestion.

What Mumble says! All of it! Especially the saving two copies of the game every time you save.

I have had single turns take close to five hours on a gig abundant all. That's true with both DA and DL. I have actually played most of the night after work before and never hit the turn button. If I hadn't learned a few management tricks on DL, I could easily see a DA turn taking 8+ hours.