I've had the same problem intermittently with 2 Dell GX280 Mini Towers.
Every time I've done the following, the problem went away for quite a while:
1. Remove all memory cards and try booting it (it won't boot with no memory cards anyway, but do not worry).
2. Insert one memory card and try booting (it may now boot ok and if you have only 1 memory card, you are finished and all should be well for quite a while).
3. Insert the 2nd memory card if you have it and again try booting.
4. Keep shutting down power and try to reboot.
At some point in the above steps, the system will start booting ok and usually stay ok for quite a while. Rarely, it might give the problem again soon rather than later and if so, you just repeat the above. I've never had the problem re-appear again quickly more than once or twice in about 4 years.
To be safe, whenever removing or inserting a memory card, be certain the power cord is unplugged and has been unplugged for a few minutes.
If upon bootup, you get a message saying that memory has changed, just hit the key you are told to hit to ignore the change.
I have successfully done this procedure at least 8 times in 4 years.