What you are describing is air in the system. Almost all VQ overheating issues are due to this. They are notoriously difficult to bleed.
The VQ in a Z/G (I believe) has a bleeder valve T'd in the hose before entering the heater core. Most people do away with that when they swap which makes them harder to bleed.
What I did on mine is T in a bleeder valve in that smaller hose that runs from the T-stat to the larger upper return hose on the passenger side. I put it all the way up top, i.e.-where air wants to escape from. You don't necessarily need that, but it helps relieve air sooner.
What you really should get if you don't have it is something like this:
https://www.amazon.com/OEMTOOLS-8700...BE2GQEJFZNFMFR
And get the front of the car nicely elevated. And, be patient. You may have to cycle the engine a couple of times before the t-stat sees the temperature required to open.