Getting 0.5 mbps on a 4 mbps plan (its more complicated than it sounds)

I have upgraded to a 4 mbps unlimited plan from 1 mbps from the same ISP. Since then I enjoyed speeds like 3.2, 3.5 etc etc never falling below 3 mbps (tests done by speedtest.net) for like 2-3 days or so. Then due to construction or some other underground work..the underground cables got damaged and many people didnt get internet connectivity ..my ISP fixed the issue in like 12 days or so by repeated complaints by frustated users (including me). Then i thought that i should also fix my telephone also (provided by the same ISP) there was a problem in the telephone jack or whatever its called (it looks like a LAN port) I have 3 of them in my house in different rooms and only the one which i used fro internet was working. So i called customer care and they sent a technician. He came and said that there is a issue with the underground wiring IN MY HOUSE. So he said that fixing the underground one would take upto a week and suggested an alternative that is a direct line straight from the base of my apartment to my apartment (2nd floor) so i chose that and he did that in abt 30 minutes and boom! i had internet and in the same speed as before (3-3.9 mbps) I had that speed for like 4-5 hours and then when i plugged in my flash drive (which i had given to a friend of mine) and suddenly the speed dropped to about 0.1 mbps. I got scared that its a virus but i dont think it was (i performed a full system scan by quick heal 2 times also by avast free) so i resetted the router back to factory settings and then set it up. After all of this i got internet but it was about 1=1.5 mbps and after 1 day or so the speed dropped to about 0.5 mbps (i didnt used it so i still had about 10 gb left) here's a screenshot - http://sdrv.ms/1efm0fV showing the attenuation/snr values and some other important info btw my router is a D-Link DSL-2750U.

Sorry for this long question but i am too frustated and handling this speed for about 15 days and now i have given up and i know i wont get any help from the ISP cause you know its a govt. company so they really dont care if they lose some customerrs, they still will get their full salaries. Thanks!