Oops, I forgot to mention what other things I did while I had the top-end apart:

New three-core radiator, new plugs (ngk) and wires.

I'm pretty sure I got all the vacuum hoses back on right. I wasn't so sure about 2-3 of them at first, but I think I got them figured out.

Question: What's that single wire (I think it's a single wire) sensor that sits on the passenger's side of the intake manifold (right next to the head)? There's a coolant tube that runs below it. I'd like to test that circuit, but I don't know how? Edit: I think it's for the temp gauge.

I really believe this problem is in the cooling system, but I have no idea how.