I've seen this posted by a couple people now with gigabyte boards and figured I'd chime in. I'm having the exact same throttling issue. I have a corsair h60 cooling my apu and load temps never exceed 63C on stress tests. VRM temps have never exceeded 57C. I can overclock my cpu without a hitch. The problem comes in when I touch the gpu. Any attempt to overclock the gpu results in massive throttling on both the cpu and gpu side. Even a 1 mhz bump (i tested this at 1251 mhz) while leaving the cpu stock results in throttling. The cpu will drop down to 500 mhz and the gpu will drop to 200 mhz for 1-2 seconds before spiking back up. It does this every 15 seconds or so in benchmarks, stress tests, and games. I figured it was my motherboard as I originally had a budget ab350m-ds3h, so I returned it today for an ax370 gaming 3. Same exact issue. I've sent in a support ticket to gigabyte. This issue seems to be tied to their motherboards. Only fix is leaving the gpu stock which stops the throttling until Gigabyte or AMD can figure out there is a real problem here.
get a discrete video card which will perk up games nicely
the rx 570 would do nicely