Problems after new graphics card

A week ago I bought a new vga and a power supply. At first it was good for gaming, for example I could play AC: Black Flag in ultra settings. Only problem was after I played games, the screen was flickering. I checked if it's seated well. Yes it's seated. Tried couple of different drivers. Flickering is still happening, though this is not the real problem. Real problem emerged last night: overheating way too fast. It's like I play AC, and then it's getting very low frame rates all of a sudden, and what's more weird is, when I alt-tad out and in again, even this process is just one second, the games goes back to normal. So, I checked the temp. This low frame thing happens when the card reaches 80 degrees, which is not normal in the first week it didn't go beyond 60-65 degrees. Anyway, when alt tab to get to the dekstop in 3-4 seconds it downs to 60 degrees again.

Things I've done already:
- Scan with ClamWin Antivirus (also for malware)
- Tried different drivers
- Defragged
- Checked whether there's some kind of connection inside the case

System:
ASUS P5P43TD-USB3
Intel Core 2 Quad Q6600 2.4GHz
4 GB DDR3 1333 RAM
Sapphire R7 260x 2GB DDR5
Western Digital WD5001AALS
Corsair VS550

Update: I have deleted the driver using 3d guru driver uninstaller, and installed the last driver again. It did work flawlessly even when the temperature over 80 degrees. Though when I restarted the computer, lagging happened again. So, it's not connected to the temperature.

Update: I has been almost 2 weeks now, no one yet helped me. I am still having the problem. Flicker I experience is horizontal shaking. It's minimal though sometimes it gets too tiring for my eyes. There also another thing I think it may be connected to the problem. Output of the new graphics card is DVI though my monitor only supports VGA. So' I used the DVI>VGA converter which came with the graphic card. I tried enabling v-sync, it didn't change anything. The games still lag from time to time, and still alt-tabbing fixes it temporarily. This I think because of low RAM. Games like Watch Dogs and Far Cry 4 has this problem. After playing it for like 2 minutes, I see the hard disk led is on' not even blinking, it's on non-stop and then the lags start. But before' there was only flickering not the low frame rate. I don't know these are all connected or not. If somebody helps me, I'd be grateful. If somebody can evaluate whether my low ram theory makes sense or not, i'd be also happy for that. Thanks in advance.