Does this occur right after startup or is the engine at normal operating temperature when this happens? If right after startup the engine hasn't warmed up to normal temps so it can't actually be running hot. In that case I would look toward the sending unit or even the gauge.
You said at highway speeds it runs normal so it can't be the lower hose collapsing under vacuum...the upper hose is likely not involved in the problem.
There's a possibility there could be an air bubble in the system that needs bleeding out. You could also have a sticking thermostat, regardless of how old it is.
From your description I would think the problem is sending unit/gauge related. If the sending unit wire is that close to the exhaust manifolds/headers, it could be burned through and maybe grounding out at times.
There are ways of testing the gauge...do a web search and find the procedure if someone here doesn't already know it.