sounds like intermittent/pending failure of components on the video card. Make sure it is seated well in the slot--sometimes you get heavy oxidation on some of the contacts and removing/re-seating the card can help clean them up a bit and correct some flaky signaling issues. Also, if you have done any clock tweaking, may need to back of a bit--what may have been stable in the cooler months may now be marginal that ambient temps are rising with the the warmer weather setting in. Note that some cards are slightly overclocked out of the box to give hem a competitive edge. While it may say they are at the manufacturer's defaults, they will actually be slightly over the reference clocks, which are what were the intended settings for the original design of the card.

Lastly, check the cooling fans/heatsinks and make sure they aren't full of lint.

***CAUTION: DO NOT cram a normal vacuum cleaner hose in there to suck out the dust and lint.***

I know many who think about doing that, but you risk serious damage if it is too powerful, and most home vacs are. They sell low powered ones specifically for use on PC's, but sometimes the ones you plug into a cigarette lighter in the car are low enough to be safe--you don't want to risk sucking up wires and such, which unfortunately is stupid easy to do if you aren't careful. Many heatsinks can be cleaned up enough just by running q-tips down their grooves though. The lint is often charged a bit with static, so it will clump up pretty well once you get the cleaning started.

Oh yeah, and while your case is opened up, boot the system and verify your case fans and such are spinning up to decent speeds as well. Not uncommon for the bearings to dry out over time with some designs and they start to lug a bit--may even groan a little too. You can often peel back the center sticker on back of the "motor" part of the fan (carefully, want it to stick back on there or you may need to get creative with some tape). In the center there will be a simple rubber cap you can pop off and then put a drop of sewing machine oil in there, press the cap back in, and stick the sticker back down over it. That should get them spinning up to speed again for a little while. If you start having to do this often, time to order replacement fans.