Computer slows gig speed download to 100 mbps when voice service is activated

I have what I think is a real head scratcher.  I have looked for answers and found none so far.  We have had Triple Play for about 7 years and recently upgraded to gig speed. I ran an xfinity speed test as soon as I installed the modem (cable/modem/router combo) and got a 935 mbps download speed.  I realized the next day that the phone wasn't working so I called xfinity and had the phone re-activated.  A day later when I ran a speed test I got 123 mbps download speed.  After several calls with xfinity a service technician came out and checked all of the interior/exterior cables and said all was well so he changed the modem.  Once again I got 935 mbps download speed. Right after he left I tried the phone and it wasn't working.  I called xfinity, got the phone working, ran a speed test and speed was 85 mbps.  Xfinity sent out another tech who attached a cable to the ethernet cable and showed me the ethernet is pulling a gig.  As an additional test I hooked up a laptop and got over 900 mbps.  He did not know why my desktop computer is slowing the speed dramatically when voice is activated and I have scoured the internet and could find nothing. If anyone has figured this mystery out or even has some things I could try, please let me know.  Thanks to all.