New Photo App on iMac deranges screen when cropping

Hi All,

I have a 20-inch early 2008 iMac, using OS X Yosemite 10.10.3., processor 2.4 Ghz Intel Core 2 Duo, memory 4 GB 667 MHZ DDR2 SDRAM, Graphics ATI Radeon HD 2400 XT 128 MB

I just updated to the new Photo, which seems OK for organizing (but maddeningly won't sync with my Flickr account any more, another issue).

But I'm having serious problems when cropping photos with the new Photo app. If I have no other apps running, it will let me crop 2-3 photos successfully, but then with the 3rd or 4th photo, the screen becomes unreadable with a random pattern. The rest of the computer seems to operate fine, if perhaps a bit slowly, but I can't see what I'm doing. For example, I can use the arrow keys to progress through my photos, but all look messed up. If there are other applications open, this problem will happen when I crop just one photo. Quitting Photo doesn't always work (even as I type this bits of random groups of pixels will flash across the screen, as I haven't restarted since the last time it happened), and I have usually had to restart using the power button on the back (because I can't see any of the items in the Apple menu in order to select the right item). Once I've restarted, things work perfectly fine until I open Photo again and try to crop photos.

Is this a Photo bug that will be worked on? Or is this a sign that my processor is too slow or my memory too small? Or is this a logic board problem?

I've added a couple screen shots below as examples.

Thanks for any advice.

Rich