Explain an oddity with coax internet signal loss
How hard is it to replace the cable from the first splice point all the way to the modem?  

I'd start by replacing that cable, then cutting off and re-terminating the connector on the cable that enters the house.  Also replace any couplers that you are using.

Part of the problem is that the center conductor of the coax cable is untreated copper.  It oxidises over time.  Same with the tin-plated conductors inside the couplers.

Where I live, the cable company will send a competent tech to do all of those things - the customer doesn't have to figure this out themselves.  Even better - they check the actual signal levels entering the premises.  They will replace the cable that enters the premises if it even looks bad.  

I've also seen them spend a couple of hours tracking down a bad splice a block away - and fixing it.