It's an electrical problem, aggravated by heat. If it was
coolant in the cylinders it would never start. Ever. Plugs would be
shorted.

Turning the headlights on while trying to start it will tell you if the
starter is drawing properly; if the headlights dim way down when the
starter runs, it's drawing well and the problem is a dragging starter
or some mechanical engine failure. If the lights don't dim much, the
starter isn't taking what it needs, and you have to look closer at all
the connections. If the connections are all new and clean, and the
solenoid is new, like you said, then it's likely an internal starter
problem like a sticking brush. There are four brushes, and if one
sticks the starter has only half its torque. Sometimes whacking the
starter will seat the brushes and it'll go.
Heat has some stupid effect on the situation, and it's common in
some GMs. The heat may be coming thru the starter mounting, not from
the exhaust.
Where is that ground cable attached?
Dan