Motorola MB8600

Purchased a new Motorola MB8600 two months ago to replace an old modem. Things worked great for about 6 weeks, then everything slowed down. When it was first installed I was getting pretty consistent 500Mbps which is what my plan was. In the past few weeks the download speed dropped to 30Mbps.  Went though online support over and over again.  Reset modem, power down modem, they would send resets.  So finally a tech came out today.  Put in an Xfinity modem and everything is working again.  So did I have a bad modem or is Comcast just pushing me to their rental?

Update:  after more problems this morning and another hour on phone support I finally bypassed my 9-port signal booster/splitter...Bang!  everything working perfectly.  The splitter/booster needs replacing.  First try for a tech to come out was 2 months.  After insisting this wouldn't work, they gave me two weeks for a tech to come out.  So now my original MB8600 modem should be put back in place, take their modem back, get the spiller replaced...