Team Fortress 2 "lerp" lagg

Hi!

So I have this little problem, when I and my girl friend play TF2 (we are ofc in the same network so we are ofc using the same internet connection) we get these epic freeze-lag-slaps and when the lag occurs "lerp 100.00ms" flashes yellowish. I have goolged it and found a few sites where ppl say that "cl_interp" should be set as low as possible (most say that ~0.03 is the lowest they can go) but when I lower it everything just lags non-stop and when I rise it the lags are longer.

So what to do? -any ideas about what would make everything run smoother?

What ISP are you with? and what country?

ISP? -Finland.

Here are a few speed test taken when our connection has it's most epic moments:
http://www.speedtest.net/result/1570260655.png
http://www.speedtest.net/result/1569446790.png
At the best the DL is @ ~1.8Mb/s and UL @ ~0.9Mb/s (ping 10) but at average is 1.5/0.8 (ping 15-20)