Is my graphics card failing?

I have a 24" aluminum iMac 8,1 - 2.8 GHz, 4 Mb RAM, Model A1225, EMC 2211,

Very recently my Mac started having major problems related to freezing and screen fragmentation and flickering. Windows become mangled and off screen. Even menu text looks a little weird. Fans/temps are fine.

Trouble first seen during some app updating and deleting, yesterday, but I'm not sure if this is a coincidence.

Reboots seemed to help, but it got worse, now starts before boot up completes. The computer is basically unusable at this point.

Graphics card is NVidia GeForce 8800 GT. Hard drive was replaced a couple years ago to a 2 TB SSHD. The HD looks fine by Onyx and Disk Utilities. Avast antivirus is clear. AHT only shows 4MOT/1/40000003/HDD-1374, which is my HDD fan speed.

I'll try safe boot tomorrow, but I wanted to see if anyone has seen this, is it an obvious graphics card failure? I'm running Yosemite, and was considering El Capitan, but I wasn't ready to pull the trigger yet.

Thanks Dan. Yesterday, I did the safe boot. It shut down 3 times at about the half-way point, then finally completed. The graphics issue was still there.

I haven't been able to figure out how to post a screen shot. I'm not sure what/where the media manager is, and dragging or pasting is not working.

After a fourth safe reboot, the desktop appeared normal. I took the opportunity to do a limited time capsule backup on a spare drive. I adjusted the energy saver so that the hard drive wouldn't sleep, and the monitor would sleep after 1 min. I periodically woke up the monitor and the backup was proceeding normally for a few hours. When the backup appeared to have completed without incident, I switched the energy saver to the default settings. The next time I checked, the screen was completely screwed up. I have been snapping pics, so if I can figure out how to share I will.

As far as the fans and temps, I originally had the SMC fan control. Everything appeared fine. I switched to Macs fan control (I can't remember the exact name). It also showed temps to be ok and all fans working. I have taken the computer offline presently, so I could set up a laptop station. I may try the TG Pro as suggested, but I think the fans and temps are not the problem, although the issue may get worse as the system warms up.

From what I've seen so far, the problem developed quickly, although not instantly. It started a little glitchy, and within a couple days became almost constant. It did not appear to be affected by a safe boot, so possibly not related to software. It seems to cascade to a crash when something is done that requires higher graphics processing. The hard drive seems to be ok. I don't know how to test further to be sure what the problem is, but I'm considering plopping down $200 for another NVidia card and hoping that swapping it out will fix the issue. I think I remember that I installed the current card shortly after getting the computer in 2008. Do they have a life expectancy? The computer has been vacuumed fairly regularly to clean dust from the screens, although I can't vouch for inside recently. The issue occurs upon the first bootup in the morning, so I don't think temps are the ongoing cause.