Issues with upload speeds dropping from 300 Mbps to 40 Mbps.

The status from the Xfinity supplied modem and my 3-way splitter. I hope this helps.

This may or may not be the root cause of the problem (YMMV) but it should be addressed regardless. 

The upstream power is on the high side and it may be intermittently fluctuating even higher to out-of-spec levels. That can cause random disconnects, spontaneous re-booting of the modem, speed, packet loss, latency problems, and the un-bonding of channels.

First. Is the modem connected to the -3.5 dB port on the splitter or a -7 dB port ? If it's on a -7 dB port put it on the -3.5 dB port and see if the connection is better. Here's more stuff to try. In an effort to try to obtain better connectivity / more wiggle room, check to see if there are any excess/unneeded coax cable splitters in the line leading to the modem that can be eliminated/re-configured. Any splitters that remain should be high quality and cable rated for 5-1002 MHz, bi-directional, and no gold colored garbage from Home Depot, Target, Wal-Mart, etc. Splitters should be swapped with known to be good / new ones to test.

Also check the coax cable for any damage such as cuts, nicks, abrasions, kinks, sharp bends, etc.

If there aren't any unneeded splitters that can be eliminated and if your coax wiring setup can't be reconfigured so that there is a single two-way splitter connected directly off of the drop from the street / pole with one port feeding the modem and the other port feeding the rest of the house/equipment with additional splits as needed and you've checked all the wiring and fittings for integrity and tightness and refresh them by taking them apart then check for and clean off any corrosion / oxidation on the center wire and put them back together again, then perhaps it's best to book a tech visit to investigate and correct.

Thanks for your help. I have remove the 3 way splitter altogether and mount it directly. Would that be better?

You're welcome ! Yes. Remove it if you don't need it.