What do the modem's signal stats look like ? Try getting them here http://192.168.100.1 or here http://10.0.0.1
Please post the *Downstream Power Level*, the *Upstream Power Level*, and the *SNR* (Signal to Noise Ratio) numbers.
What is the exact make and model number of the modem ?
Is this a WiFi connection ?
I pay for 1000 Mbps and have had two visits from Xfinity servers men to try to look at this issue. I had one of them had me change my "Speed & Duplex" to 1.0 Gbps Full Duplex. That seem to help that day until the next day I test the seed and had dropped to under 100 Mbps. Now every morning I have to reset my modem in order to get the speed I'm paying for. After rebooting this morning Upload was 38 and Download 965. Now it is down to 40.9 upload and 660.7 download.
Please re-read my reply and post the requested info / answer the questions...
Is that with a WiFi connection ? If so, try using a computer with a hardwired ethernet connection directly to the device.