iMac 27" mid 2010 i5 32 gb upgrade

Likely the RAM you installed, unless you installed Macsales.com (OWC) RAM you will likely suffer problems. iMacs are VERY sensitive to using high quality RAM and Macsales.com is currently the only vendor that has RAM that meets those standards.

Also, to perfectly honest why would you invest any time or money on a 15 year old obsolete and vintage computer???

Finally, please read Writing an effective Apple Support Communities Question so we can get an idea of what the problem you are experiencing is!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

However if it is ANYTHING but OWC that is likely the problem. Easy to check, remove the RAM you installed and see how the computer does. If it returns to normal, then you found the problem.

becu_x wrote:

I do not think a free computer and ram upgrade it is such a bad deal even for an antique computer that still works perfectly

I thought you said in your original post that it "is working flowless until now."

Is it working or not?

If its main use is for a second screen for a Window's machine it is fine. However to use it as a primary computer it's way too long in the tooth to be of much use.

Unless you used OWC RAM AND it is PC10600 DDR3 1333MHz SO-DIMM then it is the wrong RAM. For your uses, Target Display Mode, adding RAM has ZERO benefits.