Mmmmm, looks like a combination of errors here, but first off is this happening to other mixed paint or just this Tan colour and have you used the set up before without trouble?
Thinning the paint to the consistency of milk is fine for Enamels which works out roughly 60% thinners and 40% paint but I would thin acrylics 50/50, what paint and thinners are you using as some combinations of paint and thinners don't like one another? It will also take several passes of the AB in order to get a solid looking colour using the above mix, don't try and apply a heavy coat as all that will happen it the paint will get moved around by the air.
The paint looks like it's splattering slightly which could be caused by too thick a paint mix or the air pulsating, maybe your compressor can't handle the low pressures. Is there a regulator on the compressor if so check the seals, check the moisture trap if one is fitted and check that the airline isn't kinked or showing signs of damage You should not have to go above 15psi in order to spray, if you are then the paint mixture is too thick.
Another possibility is the paint/thinners mix is drying before it hits the surface resulting in the grainy appearance although that normally produces a chalky texture, how far from the model surface do you hold the AB and how warm is the room temperature?
From the pics it looks like you haven't used a primer or undercoat, I would strongly advise using an undercoat at the least (overall Lt Grey Matt is what I use) and wash the whole kit in warm soapy water using a toothbrush to scrub off any oil residue from your hands/fingers.
You say that you've replaced the spray nozzle, did you also replace the needle any slight damage to the point of the needle will greatly affect the results and check to make sure the nozzle is seated right.
Process of elimination which will take a bit of time. Need any more help feel free to ask