Why do I have very low FPS with Gigabyte Radeon rx 550 2GB GDDR5

Jul 2, 2018
hello, couple days ago i got my nvidia geforce 630 gt gpu switched to radeon rx 550 due to mine frying up after long years of use. I was pretty happy that i got a new gpu and i thought i might be able to play some games that i couldn't with nvidia, but i was very wrong. For starters i tried counter strike 1.6 (old habbit) but i was surprised that i had stable 100 fps for couple minutes of game time and then suddenly my fps would go from 100 to 30-40-60 or less and the game itself had a bit of lag/delay. Then i tried something which I never thought I'd encounter, i tried oldschool runescape which requires a java and it would run probably better on a 10 year old computer then it runs on my current pc now. The game itself has 50 fps max, but in my case i have alot of lag spikes/freezes and i see my fps staying at 20-30 with my pretty decent pc i guess. I'm not really sure whats the problem why am I having these low fps on old games, i dont even want to check what my fps will be on games like csgo,fear 3, alan wake etc.. where with my previous nvidia 630 gt gpu i would play these games with about 80-90 stable fps.

pc specs:
intel core i3 -4130 cpu @ 3.40 ghz
4gb ram
64 bit operating system
windows 7
directx 11