Strange Overheating Problem

Hi guys, I have an odd temperature issue with my 318 poly. The engine has been completely rebuilt, bored, decked etc, new crank, pistons, cam pretty much everything. New waterpump, original 22" radiator recored, with a fixed 4 blade fan, no shroud, 16 lbs cap. I originally had a 160° thermostat in it. When running it in, the temps were too cold, showing about 150° on the new electric Autometer temp gauge. I swapped in a new Mr Gasket 180° thermostat and everything was sweet, running at dead on 180° in traffic or on highway. It would warm up to over the 180 mark, then come back down to 180 and stay there. During this time it puked a bit of coolant a few times, especially if I'd been running it hard shortly before shutdown, so I assumed it had found it's own level.
Last week it started running hotter, like 210° hot. This coincided with it being pretty hot and humid here, say 95°F, but then it's always warm, even in winter it's 75°F. This seemed to happen overnight - one day 180, next day 210.
It runs at 210° regardless of whether in traffic or on highway at 70mph. I've drained a bit of the radiator and the tubes seem clear. It also run at 210 a few days later when the temps were cooler (about 85°F).
I was thinking waterpump or stuck thermostat but wouldn't it keep getting hotter and not stay at 210°? Could the gauge be dodgy? I've also noticed the waterpump has a slight movement in it when I grab the fan and rock it fore/aft which is a concern for new pump.
Also, I topped the coolant up with 900ml of water (nearly 2 pints) and while that didn't make a difference to the temperature it didn't puke it out, which I thought it definitely would at the hotter running temp.
I done a fair bit of research on this and realise ignition timing, fan shroud etc all might help, but nothing has changed on the car recently except the temp it runs at. Any ideas?

Mr Gasket thermostats have issues, often losing travel, creating a restriction. Try a EMP/Stewart.

https://www.summitracing.com/parts/emp-305/overview/make/dodge

Does Stant make a stat for the Poly? If so that is all I run is Stant.

Thermostats are inexpensive, swap it out with your known good working thermostat. All the thermostat regulates is the minimum operating temperature.

Thanks for the replies guys. I took it out last night for about 35 mins, mixture of freeway cruising, getting on it, and town traffic, long idling at intersections, didn't budge from 210°, didn't puke when I got home, and didn't feel especially hot when I popped the bonnet after it was switched off. The front of the radiator felt warm all the way across. I'll swap the thermostat as a start but I'm wondering about that gauge. Shame Mr Gasket has succumbed to the cheap production costs in China, I thought they were a good brand.

First thing I would do is get an IR thermometer gun and verify those temps, before changing anything...autometer gauge should be good but you never know, could just be electrical/grounding issue. Based on your description it seems like the thermostat is failing. It may not be stuck completely closed; it's possible to be partially open just enough to cool it to the the 210° you're seeing. But double-check that gauge first..
For thermostats the US-made Stant Superstats have been the best I've used (it gets hot where I live..)

What type of fan are you using? A temp gun is your friend!

Amazon. $15.99. As was said, this will let you know if your gauge is accurate. Really handy tool. https://www.amazon.com/Etekcity-Las...ocphy=9032520&hvtargid=pla-313800562779&psc=1

Or a local Harbor Freight.......https://www.harborfreight.com/catal...,f,EAFeatured+Weight,f,Sale+Rank,f&q=temp+gun

Smokin deal for the guys that want to be Jackstand racers

If a new stat doesn't help, a fan shroud might be worth a shot. Worked for me.

P.S.
I too am running Autometer (mechanical) gauges and they seem right on the money.

Gauges can be off,but in your case I feel it's something mechanical as you changed nothing before the problem started.You can change the stat and see if that helps,but before you start throwing parts at it the problem deeds to be diagnosed.Maybe there is an Autozone or the like near you that loans parts or does a free scan on the newer cars.If so,,,,,drive the car and pull in at 200* + and have them shoot the temp gun at the upper & lower rad.The upper hose and the top of the water pump.Check your oil to make sure no coolant is leaking in from a bad head gasket.

If it only feels warm under the hood, it's probably the guage. At 210 degrees it's pretty hot and you're not going to be able to put your hand on anything. Get a IR gun first. They are really handy. Maybe consider a mechanical temp guage instead of the electric.

Quick update, it was the thermostat. It must have been only partially opening. I swapped in my old 160 degree job (don't know the brand but is stamped "Made in USA", which is reassuring). Runs great now, perfect for the Australian summer! Thanks for all the suggestions.

Made in USA is always a good sign!