They might not have really fixed the issue properly. Or it could just be a perfect storm of multiple problems, and they only solved one of them.
Note also that if you're on a wireless connection, you might be in an area of your house with poor reception. Or the modem itself is placed poorly. Again, this is assuming your modem is a modem/router combo with wireless connectivity. But to confirm that this issue is real: I just recently had to move my router because one of my machines was getting 80% packet loss. Test your connection with something like http://www.speedtest.net
to confirm that your rates are reasonable, but more importantly, consistent. If you're seeing lots of spikes, it's the wireless connection (or just the modem itself). Doing the speed test would help even if you don't have wireless, as the spikes would be indicative of network problems either way.