I've been pulling my hair the last couple months trying to figure out why this is happening, but I found some useful links that may help you!
https://www.reddit.com/r/Amd/comments/5om2hg/please_fix_or_at_least_acknowledge_the_150300_mhz/ - The LoccOtHaN regedits
I originally thought it was with Dell, but it turns out its a driver thing. I turned off Sleep mode in the power settings as well, so I hope you arent too attached to them. I didnt test it with sleep mode.
As always too, use AMD's latest drivers if you can, becausen I found Dell's to be rather lackluster.
Please give feedback and do let us know if it works so others can get this information as well. After I switched those around, life was fantastic seeing the card actually run!
EDIT: Also, in msi afterburner, turn on "Unified monitoring" and remove any other hardware monitoring applications. Apparently they cause conflicts as well.
EDIT EDIT: Very sorry for the muliple edits, but I would also check your thermal paste in the computers. I originally thought when I first got the 7720 it was an overheating issue. Even though that wasnt the solution, Dell's application of the paste made me want to cry it was so bad.