[ HELP NEEDED ] Drop on frame rate as I progress

[Rucifer54]

So I'm a new warframe player. My PC specs meet the minimum requirements. When I first started the game, I optimized the display and graphics settings and the game was running pretty smooth with no lag or problems whatsoever, in solo OR online matchmaking. But as of recently, when I start the game, it runs normally as usual. But as I progress through any mission a bit, it starts lagging to the point where it's almost unplayable, I've updated all my drivers and tried lowering the graphics even more but to no avail. I went to the audio settings and stopped reverb as I saw that it should help, somewhere. I've also disabled 64 bit mode from the launcher settings, but the problem still persists. Now I'm not at all sure what is causing the problem and a little bit of help would be greatly appreciated. Restarting the game seems to reset the problem, the starting is pretty ok, but after a little progress game starts going haywire
Ok so new information, I just noticed, that during my playthrough, ram usage just continues to rise over 7 times the initial usage. That's the max usage that was shown, and something weird happened yesterday, I was playing defence with a friend so I couldn't just leave the game because of lagging, and I continued playing, and after about 15 minutes of playing with insufferable lag, the game just went to perfect smooth again. But the ram usage was still very high up

:: My PC specs are ::
8GB DDR3 RAM :: Nvidia 640 GT :: Intel Core i5 3470  @3.60GHz ::

Edited March 17, 2017 by Rucifer54
Just noticed new facts