The first thing I would suggest is checking the fans. If the bearings on one of the fans is shot then the card will always downclock. If you bought your card from a Cryptocurrency miner, then keep in mind that the person who was mining was running the card 24/7 with the fan speeds at or near 100%. I had an R9 390 that was having this problem and the fix was to buy new fans.
I have an MSI Gaming X RX 580 and after some time it developed issues with maintaining clock speeds. While the temps never got too high, it just wouldn't overclock or even maintain the 1366mhz base speed. Then I took the heatsink off the card and discovered some kind of foreign material between the heatsink and GPU die.
The card may downclock if there are no thermal pads between the VRM's and DRAM dies.
What I would suggest doing is checking the overclock with programs like Furmark, Unigine Superposition, and 3DMark TimeSpy. If the card can't maintain the 2ghz clock speed you put on it with those programs then it's a problem with the card, heating issues or the drivers. I've seen certain scenes in games cause a card to downclock. What's more is you have the 3gb model so it might not be a bad idea to track Frame Buffer/VRAM usage when the card lags. As the Anoymous poster said, you might need a better card.