Mid-2011 iMac Down For The Count

[Thrift Store Scott]

Buzz wrote:
Start up from DVD or external HDD, and what happens?
==

When I try to start up from DVD, I get this:

"Three beeps, then a 5-second pause, repeating
The memory in your Mac didn't pass an integrity check. If you added or replaced memory, make sure that it's properly installed."

I haven't touched the memory at all, but from day one the startup bar has always paused for a second or two at the half-way mark before progressing, and this pause got noticeably longer when I upgraded to the more RAM-intensive Sierra OS. The half-way mark is where the startup progress bar stops entirely now, so maybe a stick of my RAM has been on and finally completed a long, slow death spiral.