Connectivity/speed issues

Hi,

This has been the absolute worst customer service experience I have ever had.  Signed up for 30 mbps internet.  Never had an issue. Was offered an upgrade to 100 mbps at a good price so I signed on. Ever since upgrading my speed rarely gets over 24 mbps, often drops to 0.6 mbps and upload speeds never exceed 4.5 mbps (supposed to be 5-15 mbps). Originally found that restarting the modem would get speeds to bump up to 100-120 mbps, but would drop back off within 20 minutes. Replaced that modem and have had the same issue. Had 2 techs come out. One replaced line from main Xfinity line to house, the other replaced the drop from my attic.  Both said the problem was with the main line but that their line replacements should "help" but not completely fix the issue. No one will come out to address the main line. Third tech came out, said all lines are good the problem is the modem, will you consider renting one of ours. No, I have bought 2 modems and previous two techs said the problem was with the outside line. "Sorry sir, this is all I can do". Have used tech support through netgear. They sent another modem (3rd modem!!!). Same thing, same problem. Go to Comcast Customer Service Center - "all I can do is schedule another tech to come into your home". In addition, someone was supposed to raise the main line (Xfinity's line) because I can stand flat footed and almost grab it (I'm 5'6"). How do I speak with someone in management that can get something done? Could not get event log to paste here, but a lot of 'Critical 3 SYNC Timing Synchronization Failures' and 'Notice 6' and 'Warning 5' issues. Thanks for any help.