It doesn't act up until the engine has been warm for quite some time.

Also I don't normally buy repair parts until I have checked the old
ones but from my experience it acted as though this was the problem.

I have 97 960 wagon with a 2.9l six. I believe the temperature sending unit needs replacing. The temp gauge will go all the way to hot on occasion then immediatly return to normal, or will vary from hot to normal at an etremely quick pace (less than a second). I have purchased a new part but cannot ...