war

Heat + time = degradation. Watching a video technically shortens the card's life span, but most electronics will fail due to manufacturer defects before actual electromigration effects. I incidentally ran 7 5850s when GPU mining for roughly a year, and none of them failed even though they ran 100% 24/7 overclocked. The important thing to do is monitor temperature of the cards and try to make sure they're never going above 80*C. Work load and degradation don't go directly hand-in-hand -- it's the heat from the work load which you need to watch.

There are a few ways to lower heat... the easiest for enclosed computers is to just take the side of the case off. You want to clean it every couple months, and if you have heat problems in Summer, might want to consider undervolting it. You can often undervolt the card 20-30mV without needing to underclock it for stability, reducing heat (extending the card's life), increasing efficiency, and not impacting performance at all. Alternately, can use a box fan, or just not run it while it's hot out.

To prevent lagging, you want to lower the intensity (or whatever sounds similar in the mining program of config file, in some of them it's called "PRIORITY" or something else) to somewhere where the graphics aren't "stuttering" but you're still getting a good hashrate. Incidentally, this will probably also significantly reduce the heat output.

Your miner is probably mining on the CPU instead of the GPU. (It's been a long time since I used GUIMiner -- assuming you have up-to-date drivers, you should see options to mine either on the i5 or the AMD card in the "dashboard" tab)

Or you're mining a Scrypt coin like LTC, in which case, your 5850 will mine dramatically "slower" than mining something like BTC.

Your doing nothing wrong, you need another rig.