EVGA 550 G2 owner here. I haven't had this problem, but I feel like the eco switch wouldn't/shouldn't be tied to anything but the fan controller. Makes me wonder if your eco mode isn't working properly and heat is building up and the PSU is shutting itself off before the fan comes on? I know the Superflower (2 series) models leave their fans off longer than the SeaSonic (S series) models, but either model should turn the fan on before temp shutoff occurs. AFAIK, the EVGA platinums are all P2, aka Superflower.
Main: i5-3570K, ASRock Z77 Pro4-M, MSI RX480 8G, 500GB Crucial BX100, 2 TB Samsung EcoGreen F4, 16GB 1600MHz G.Skill @1.25V, EVGA 550-G2, Silverstone PS07B
HTPC: A8-5600K, MSI FM2-A75IA-E53, 4TB Seagate SSHD, 8GB 1866MHz G.Skill, Crosley D-25 Case Mod