Strange issue happening and looking for some guidance. A few months ago I purchased a Refurbed i5 5K 27" iMac from the Apple Store. It is the base model so only came with 8g of RAM and the 1tb Fusion Drive. All runs perfectly UNTIL I install additional RAM. I copied and pasted the RAM specs directly from the Apple tech notes to do my search. Initially I installed 2 16g OWC modules. They worked for about 2 months then seemed to cr*p out and the computer would not even start up. I removed the OWC modules and everything was back to running and starting OK. So then I bought another brand, Timetec, I believe. Installed those and worked great for 2 days. Then I shut the computer down and it would not start back up. Removed those two RAM modules, and as expected, the computer starts and runs fine. I even removed the Apple memory and installed the 4 16g OWC and Timetech modules, again, no startup, nothing, nada. It’s not like the computer is slow, but I’d like more RAM. I am using an external ssd boot disk for startup and applications. Any thought or others’ experience with this issue? Thanks.