AC Adapter & perfomance issues

Hello.

Dell Vostro 5459.

First, my wife complained on diagnostic screen appearing during system start now and then. Haven't yet seen it myself though but from what she said the screen is about well known "the AC adapter type cannot be determined..." problem. Everytime she just skipped it and all went well.

Then we noticed random perfomance drops. In Windows event log I found some warnings about CPU working on lower frequency. CPU-Z confirmed that showing only 400Mhz (it's an i3 6100U 2.3Ghz).

System restart doesn't solve the problem. But simple AC adapter unplugging works great. Freq boosts to the normal at once and then we can just reconnect the adapter cable back and work for another 2-3 days untill it occurs again.

Some more info: right after purchasing the device (about 3 months ago) I replaced HDD for SSD, flashed latest BIOS version and did a fresh install of Win10Pro Anniversary. Forbad OS to update drivers, did it via Dell Support Assistant instead.

AC adapter and coords ain't damaged, always properly connected and I tried different wall outlets (who knows) Battery charges well. The Laptop is used for... everything. Gaming included. Not sure whether perfomance drops happen only during high load (e.g. game) but HWMonitor never showed more than 67C and judging by tactile perception of the bottomplate temperature is far from CPU throttling limits and It wouldn't be solved by simple reconnection of the power cord.

Forgot to mention, AC adapter produces high pitched noise. Not that loud but still a bit irritating. Dunno whether it may cause the problem.

So that's it. What else can I try to know for sure what it is? Is it a warranty case? Any pieces of advice will be appreciated.