Intermittent connectivity problem: MDD Timeout, Sync timing synchronization failure, No unicast...

In case it's helpful, here's what I'm seeing in terms of connection:

Downstream - SNR looks ~32dB right now, pretty sure it was a little over 50dB when I looked earlier today. The line to the street isn't great, Comcast put in an amplifer a few years ago which helped a lot.

Upstream

Thank you @CCAndrew! Much appreciated.  I'll call for a tech if I have to, for now considering it last resort.

Sorry for the dumb guy question, but can you let me know what it means to measure it at the block? For example I could bring the router down to the basement & plug it in where the line goes into the signal amplier.

Meantime I swapped out a splitter for a coax connector (unknown quality, but it's not gold), and I think (?) I've made things a little better, by the numbers pasted below.

With some work I can probably move the cable so that I can connect it directly to the modem. Or I can purchase a new connector of known quality.

The connectivity problems are hours apart, and I may not always detect them. So it'd take me a few days to determine whether what I've done so far, actually made a difference in the outcome.

Thanks again for any advice.

Here's the new numbers:

Progress!  I was able to remove another splitter, and a coax connector+ cable, such that there's now a single cable between my modem, and the signal amplifier for the incoming line from the street.

Now I'm seeing better signal strength, and so far it seems like connectivity issues are resolved. Fingers crossed!

Modem is split after the amp. I just changed the amplifier to add an active return.

Old amplifier: Electroline EDA 2100, +15dB forward, passive return

New amplifier: Arris BDA-42-1-AR-R, +15dB forward, +10dB return

I also swapped out the RG6 cable that supplied power to the amplifier (dog had nibbled through to the shielding...). After checking the levels, I added a 7dB splitter/attenuator. 

I think all of the levels are now pretty close to the sweet spot. Fingers crossed for reliable connectivity.

Thanks. I moved the splitter before the amplifier. I put the cable modem behind a -7dB port of a three-way splitter. So now there's no attenuator or splitter between the amplifier and the modem.

I connected the cable TV line to the -3.5 dB port. I don't have cable TV service, so that's basically inert. There remains  an unterminated -7dB port on the splitter. I've got 75 ohm terminators arriving this week from Amazon.

The signal looks pretty good to me:

Without the attenuation from the splitter, the downstream power goes to about +5 dBmV. Upstream power goes to about +37 dBmV, if I recall correctly.

I think those numbers are out of spec, aren't they?

Net/net, I was aiming for a setup where downstream power was ~0, and upstream power was about 45 dBmV, with a good signal to noise ratio, something like 40 dB. I thought that was about ideal.

Seems to me, IIRC, Spec says -7 t0 +7 on downstream power- anything -8/8 to -10/10 is "acceptable" (but better to get below 7) and anything -11/11 and up is totally out of whack and causing problems, and SNR as long as you're over 33 you should be in good shape....On the upstream, you look to be right in the sweet spot, on 64-QAM upstream you really want to be between 35 ad 49....

Also, all the other advice is good. Eliminate as many points of failure as possible between your modem and the block and you'll be in the best shape. The numbers you mentioned before, when running straight from the line sounded ideal to me- or close to it.