Hi gyro, welcome to the forums, and welcome back to the hobby.
It sounds like your paint and thinner are incompatible. This is not as odd as it may sound.
Gunze (now known as GSI Creos) have two ranges of paint. One, packaged as "Mr Hobby" (or "Aqueous Hobby Color", or simply "Hobby Color") is acrylic and the other "Mr Color" is a lacquer based product.
The acrylic range, regardless of how they are branded, are in squat, round jars nearly identical to Tamiya's acrylic paint jars.
The "Mr Color" range are sold in round jars with a lid which tapers up into a flat "paddle" shape(this is my description anyway).
Each range has its own thinner - the stringing you describe sounds as if you are trying to use the acrylic thinner with the lacquer based paint range. The acrylic paints should mix with water, so if it's separating, it also suggests you may be using the lacquer based paint rather than the acrylic.
Check if the thinner you are using is labelled "Mr Hobby Thinner" (or "Mr Aqueous Thinner") (acrylic) or "Mr Color Thinner" (lacquer). The number (400) isn't important - this is just the quantity in the container (400ml). If you are using "Mr Color" lacquer based paints, you will need "Mr Color" thinner or "Mr Levelling Thinner" (this is the same lacquer thinner with a drying retarder added).