I'm having problems with my P3 and riser cable too.
First cable caused reboots, boots with no image, driver fails warnings, and constant latency spikes and sound stuttering.
Contacted Thermaltake, and got no response so far, not even a "Thank you for contacting us".
I'm from Portugal, and dealt with the store I bought the case from (PcComponentes, in Spain).
They sent me a new cable yesterday, which I assembled so very carefully avoiding cable strain and keep the cable curves as wide as possible.
First run on new cable: same constant latency spikes and sound stuttering.
GPU went back directly on the pci-e slot: all problems gone.
Asus P8Z77 WS Motherboard
Asus R9 390 Strix Direct CUIII
Powered by a Seasonic SS-760XP
I just sent an email once again to firstname.lastname@example.org, but I don't even get an automated reply...
Saw some post before claiming the 390 is not showing much success on this riser cables.
Should I keep waiting or just give up on this and try to return the case?