According to TechRadar, 76*C is the highest they got in their suite of tests, (Handbrake, Cinebench, Geekbench), so no it is not normal to idle at that temperature.
dream-mod, those temperature make no sense. The maximum temperatre for the 2990WX is 68C:
Unless you Disable PROCHOT (not recommended) the processor will shut down long before that. The only temperature measuring application that is valid is Ryzen Master (RM). Please post a screenshot of RM - simply drag-n-drop the image into your reply. Thanks and enjoy, John.
EDIT: My specifications and RM running Prime95:
MSI X399 Creation, Threadripper 2990WX, 3xSamsung SSD 970 EVO RAID0, 4xSSD 960 EVO on MSI AeroXpander RAID10, 1TB & 500 GB WD Black, G.SKILL Flare X F4-3200C14Q-32GFX, Windows 10 x64 Pro, EnerMax-MaxTytan-EDT1250EWT, Enermx Liqtech TR4 280 CPU Cooler, Radeon RX580, Aquantia 10 GbpS Ethernet NIC, UEFI E7B92AMS.120, AGESA SummitPI-SP3r2-1.1.0.2.
