"No were near 1Gbps")

I have been searching the Net and these forum pages for answers.
I hope someone here has a good idea!
Here is my story;
I had new service installed less than a month ago, Dec. 6th (Triple play)
1000Mbps internet, 250+ channels, and a VoIP phone line. During this time I have not seen more than 285Mbps down and 42 up. For the most part, I see between 125Mbps - 148Mbps down
And 25Mbps – 37Mbps up. Ping is between 9ms and 16ms. I’ve used several test sites including Xfinity’s.

The RG-6 coax cable comes into the house and goes directly to a power amp / splitter. From there it goes to the xFi advantage modem/router. The two RJ-45 connecters on the back of modem; One goes directly to a custom built desktop PC via a Cat 6 Ethernet cable (8’)
[All tests were made on this PC]
The other RJ-45 goes to a 16 port gigabit switch. The whole house is a Cat 5e wired network.

After the initial installer (sub-contractor) left I did a speed test. About 212Mbps dwn. And 35 up.
What a shock after getting used to 960Mbps Dwn. and 830Mbps Up with AT&T fiber.
A long drawn out phone call got me an apt. with a tech. He changed out the pwr. amp and the modem twice. Same! Next I connected with ‘Chat’ (50min) and another apt. The next tech said he can not test 1 gig speed but he is getting over 600Mbps on the WiFi. I showed him the Xfinity speed test on my PC 135Mbps. New day new chat, 45 min. later, new appointment. This third tech did indeed have a gig tester. He said “….everything is fine on my end, you’re getting about 920”. Again, Not on my PC.
I now looked into my PC. On board network adaptor is good for 1Gbps. Next I tried an Intel adaptor that I installed in the PCIe slot. Worked but no higher than 140Mbps. Tried a ‘tp-link’ card , Same!
I also tried re-booting the modem and PC several times, started and ran speed test in safe mode.

Any other Ideas???
ZAK