I can't even think of what could cause that level of slow down on the NIC. Unless it's a problem with the NIC itself, but not really sure how cleaning up software programs would see a 4x increase in speed, which then went back down to 200mbps just days later. That sounds like BS. OR a hardware issue.
If the modem/router is 1gbps, and the NIC for your computer is 1gbps, should easily see 800mbps+ Minimally should be seeing 600-700Mbps unless the ISP is the one with the congestion.
I don't have too many details because I didn't get a chance to hang out as much as I'd like. I also don't know how many of these computers were tried wireless vs wired and most people's wireless connections are shit.