Allow me to share my musings about this, ever so common, high, indicated, temperature phenomenon. I have arrived at a couple of suspicions as to culprits-- aside from the notoriously inaccurate temp gauges and sticky thermostats: 1) At low revs the water pump doesn't circulate enough coolant around the temp sensor. This situation may be agravated by the holes in the head gasket at the rear end of the block being too small and partially blocking off the coolant passages. 2) The temp sensor sits right next to the exhaust valve and its port, in a location where a lot of hot air rises. If I'm right about the head gasket holes, it underscores the importance of making sure they are large enough and line up properly with the coolant passages. I was aware of this when I put my engine together, and I did some cutting, but now that I look back, I could, probably, have done a more thorough job.
I'm sure Phil's "if it doesn't boil, don't fret" advice is correct. Following his advice, I've also advanced the timing, and that seems to have helped. With all due respect, however, I 'd like to mention that I have gone from a two-bladed fan, to a four-bladed, and then a five-bladed, and that this hasn't made a bit of difference. People I have heard from who installed electrical fans report greater peace of mind at idle and in slow traffic in hot weather, though, they never really had a boiling problem before either.
Bob S.