Will try another monitor with 60 Hz. But I don't eneble G-Sync on my XB271HU 144 Hz monitor.Connect your PC to your TV with HDMI and see if it helps with the frame drops. It could be related to your G-Sync monitor.
I have AMD Radeon HD7700 will try after all tests.Do you have any other GPU to rule out GPU issues? Spikes can be caused by the GPU randomly clocking down due to a failing power delivery system. Faced this on an R9 290 I had for a while.