CPU stuck at low speed (0.48GHz)

Laptop: Inspiron 7352 with Intel i5-5200U 2.2GHz, 8Gb RAM, Window 10. 

This laptop has worked fine for years. I swapped the hard drive out for an SSD. A few weeks later the CPU speed has suddenly locked at 0.48GHz, causing the computer to slow to a crawl. I scoured the internet and dell forums for solutions. None have worked thus far: 

1. Ran Dell Update and Windows Update to ensure everything is up-to-date
2. Ran virus checker and MS scanner for viruses
3. Power adapter is correct 19.5V original plug (some claimed this could cause this issue)
4. Tried removing battery for 5 minutes (someone claimed this worked)
5. Power plan in control panel set to Dell's. Max CPU is 100%. Cooling is active. 
6. I turned off intelppm in registry (set Start to 4). This locks frequency at 2.2GHz, so no power mgmt. However the laptop still feels sluggish. 
7. Rebooting with intelppm ON (Start == 3) the CPU frequency adjusts correctly for a while. Once it heats up it locks at 0.48 again. 

I'm out of ideas. Anything else I can try before I toss this laptop in the garbage?