Laptop starts to lag after some time

[AntiSaby]

I've HP Pavilion 15 and It's been working all fine ever since i got it last October. It started to lag weirdly earlier today. When i open heavy applications or a game, mouse cursor starts lagging, songs starts lagging while playing, everything slows down a little. It gets even worse after i unplug the charger. I can clearly notice the increase in lag after unplugging. It all happens after working for 30-45 minutes. I can't play any game either. It's not like normal lag. It's like really irregular and I can hear scratching sounds while songs are playing. After turning the laptop off for 5-10 minutes minutes and restarting it, everything returns to normal. Below are the specs:
i7 4510U 2.6 GHZ
8 GB RAM
GeForce 840M 2 GB

Edit: I don't think It's a GPU or overheating problem. After restarting i tried playing Far Cry 3 at Medium-High settings for 10-15 minutes and there was no lag at all. I tried changing power plan to a new default Custom plan with default settings and everything was still fine. one thing i noticed is i get lag spikes when i switch applications like switching from Firefox to Skype or Skype to Chrome and so on.

 

[BeefyTriceps]

Try to note down you laptops temperature and Ram usage. I recommend you download MSI after burner and youll know if the overheating and ram eating is the culprit.
i ahve the same prob as you and i haven't found an answer yet.