Sudden freeze and beeping amber led at power button

After years of faultless service  my Dell Vostro 470 desktop has recently developed alarming symptoms. It can be running well for some hours, when suddenly it freezes, some thin horizontal orange lines appear across the screen, accompanied by a blinking amber led light at the power button with a simultaneous buzzing alarm sound going off. The only way to deal with this when it occurs is to use the power button to switch off the PC.

The first few times this happened I could not even POST. Then I discovered if I turned the PC on its side and powered up it booted ok. When I put it back to its usual standing position it still continued to work without issue. I ran it through a UEFI diagnosis test and when it came to testing memory at the WTAM test, the symptoms re-appeared. I thought it might be a failing memory module, and I checked it was seated ok, which it was. Since then, it continued to run fine and the next day I tried another UEFI test, and this time all the tests completed without issue. A few more days passed without further incident.

Then today after running for several hours, I was watching a video and in the middle of it the PC suddenly froze with the symptoms as described. Again I switched off the power, opened up the PC, turned it on its side, checked everything was plugged in firmly, and it rebooted fine. Put the PC back to standing position and no further problems. It's been running another 2 hours without issue so far. The temperatures of the CPU cores are all running at 28° - 30°C. My 2 disks are not showing any issues according to SMART and running at 30° C  and 38° C respectively, the latter being the one mainly in use.

So any ideas what might be happening here would be much appreciated.