Shall I add a RAM or change the PC?

These days chrome is hogging up 1GB depending how many tabs you got open, Windows 10 uses up about 512mb while doing nothing.
4GB more won't do much harm, but it's not really worth spending much on a laptop anyway.

If it's over heating, it will cause a bottleneck with the CPU, take the back off, clean out the dust, you could try getting some good thermal paste and re-applying. Just sounds like a heat issue bottlenecking the CPU.