Looking for advise.

1994 Ford f150 5.0liter, automatic. The truck has had this issue for a long time and I am determined to figure it out. Maybe one of you fine folks could help.

The truck stays nice and cool or right at the thermostat opening point all day long regardless of outside temp when idleing or driving under 60 mph. BUT, after about a half hour of highway speeds the temps start to climb to the red zone. Changing RPM will affect the temp gauge, letting off the gas pedal will cause fluctuations. Flooring it will sometimes drop the temp down back to normal. The gauge is going up and down like air pockets or steam is trapped in the cylinders.

So of course I have replaced many parts trying to fix this:
Radiator (cheapy, aluminum and plastic).
A new genuine Ford thermostat, and cap. Also tried a 180F thermostat.
New Hi flow water pump from Summit.
New upper hose with a spring installed.
Lower hose looks good and still has spring in it.
NEW HEAD GASKETS, oh yeah that was a hell of a job. The tabs are in the front they are installed right.
Flushed coolant. Burped system a million times.

Guys, what should the flow look like when the stat is open??? Looking in the filler neck I see almost no movement but the level will change when I rev the motor. If I dissconect the top radiator hose (thermostat removed) coolant does move, although slowly.

Should the coolant blast out of the disconnected upper hose, or just a steady stream?????