As I was reading the post I thought OK, new engine so maybe not with a head gasket problem. I was thinking that there must be something you are doing with the antifreeze mix. I do a 50/50 and never had problems with hot or cold temps.
Even if it is a new to you engine I would still be doing a compression check on it just to be on the safe side. That can be done real easy too. Just take one of those mechanics gloves or latex gloves and put it on the expansion tank in place of the cap. Do this prior to starting it up cold. Then start the engine gentleman! In about a minute do you see the glove starting to inflate? Likely that is caused by the air coming in from the cylinders and thus you have a leak on the head gasket.
Food for thought or an excuse to be under the hood.
This test is a winner for Head Gasket Failures, and they can occur at any time, as I used to run a Diesel that would blow one every 6months because of the bolt issue on the engine.
The CAVEAT is that you tie the Glove tightly over the open Expansion tank.
You have to Start the engine, then REV it above 3000K for 45-50 seconds any longer and the Glove will Auto Inflate from the heat and steam of the a/f.
If the glove inflates and gives you the finger, then you have a blown head gasket. On Gassers it can either be a blown head gasket, or a leaky water jacket oil cooler.
Another indication of a head gasket failure is that your Hoses will Balloon and be hard to squeeze but once the engine stops your Hoses return to normal.
Easy to see on a running engine that is warm.
If you do have a Head Gasket failure then I would strongly urge you to install the ARP stud kit.
These kits are re-usable and they when installed usually won't loosen and take care of the bolt loosening issue that the 1.5 engines can be prone too. If when I had found this out, (back in the Pre-Al Gores invention of the internet) I would if installed these. The Head Bolts are not re-usable they are a one time use only. They also have to be re-checked for torque at about 500-1000 miles. I suspect the best thing is that I got really really quick at changing the head gasket. I will also suggest that you place the engine in time prior to diss-assembly.
Causes of the Expansion tank issues (and Caps) is two fold, a blown head gasket or Crappy parts.
I just went through this on one of my Cabriolets, where the Cap wouldn't tighten down.... new tank and cap later (bought as a pair from O'Reilly's) worked.