stabbyTearing should only be occurring when your FPS exceeds your refresh rate. It's odd that you're experiencing it with low frames. 70 minimum is actually about what I'd expect from your specs, by the way--tf2 is simply poorly optimized.
Make sure fps_max is set to "0", it can create odd "tearing-like" artifacts and there's no point in capping unless you're using Lightboost/ULMB or Gsync.
Back up your settings/autoexec and try the launch option "-default" to let tf2 auto configure itself. If this fixes it you can rest assured it's a certain command/setting or combination of commands in the game that are the culprit, not your setup.
Speaking of gsync, it *is* a very nice upgrade that would resolve tearing, among other things. If you have some holiday money left, you might want to consider researching gsync monitor options. You could sell your current one and buy a nice used display with similar specs + gsync for a reasonable net expense...heck, I'd sell you my Gsync vg248qe for what you could sell that benQ for, come to think of it :P
Thanks. For some odd reason, after a night's break, the game works just fine now.
I would also like to let you know that at high fps gsync actually increases input lag so i would not recommend it for games like tf2 or csgo (source: http://www.blurbusters.com/gsync/preview2/).