Dell only test/validates using the standard 2m/6ft DP to DP or mDP to DP cabling. Do this and see if the issue returns.

Tested with shorter cable (less 2 meters) - same issue, constant flickering. DP -> DVI (8 meters) - no flickering

Being that this 6 year old monitor has no warranty, you should stick to using the cabling that works,

PC DP out port --> DP to DVI cable --> Monitor DVI in port

I would also test a different monitor there that has DP in port and see if the same issue returns.

PC DP out port --> 2m/6ft DP to DP cable --> Monitor DP in port
PC mDP out port --> 2m/6ft mDP to DP cable --> Monitor DP in port