So, I don't know if it's a bug or intentional, but myself and some friends have all noticed the same issue. The more often you play, the more XP degrades. For example, all of our CP characters get a minimum 10x drop in XP after playing for several hours. Then if we wait a couple days, the XP goes back up, but only for a couple hours. If we wait a week, the XP goes up and stays up for several hours.
Happens both in PVE and PVP. Cyrodil dailies for example, yield 70+k XP if using XP potions. Then we hit some wall where it all suddenly drops to 18k. Frustrating as heck when you're trying to level a character. I get farther and faster if I only play once a week than if playing 8 hrs/day - (summer break

Our household has three accounts, all with the same issue. And as I said, my friends also notice the same issue. What's up with the XP degradation thing? It's like we're being penalized for playing too much?