I have adelphia internet

Signal level or noise or bad amp on the cable string, bad cable connection at cable amp,..... bad technician :(

And if you're near my house, it could be HEAT - it's like 112 today.

Why does heat affect cable modem service? (Not doubting it, I live in Phoenix and I know it does but don't know why.)

All those little electronic parts generate/hate the heat - esp. CPUs which generate a ton on their own (take a look at that fan on your CPU). That's why they run liquid nitrogen through the really big/fast super computers.

Enough heat can cause partial/total breakdown of electronics - I think my router has been overheating the last two days requiring 2 reboots.

If a varmit builds a nest against the vent for a powered distribution node and the outside temp climbs to 112, well the temp inside the structure could easily climb to a point where the electronics can't keep themselves cool enough.

Signal levels vary normally as the outside temperature changes - higher levels in cold and lower levels in hot. The reason is that the resistance or loss in the metal conductor of the cable increases as the temperature rises. If you have a -10 reading on a cold day then when summer comes it may drop to a -15. Of course, being close to the lower limit of the cable modems range, usually -15, makes it more possible that you'll exceed it during hot temperatures.

it could be the traps on the main feed.