Temperature gauge/ tach problem

So i recently did a b18c1 swap in my 91 gs, and it has been driving “fine” and i’m still working on the some little things. whats weird is i took it for a drive today and not everytime, but once in a while, i would press the clutch in and my temperature gauge would instantly go from average temp, to over heating, then when i released the clutch it would go down. I’m not sure why it’s doing this, any information i would appreciate. I can;t figure this out.