Did you try running at lower temperature (~300K)? It would be better if you could post a part of your thermo info in the mail. Furthermore, how big is your system?
About the fix, are you running npt straight after minimization? Try running NVT first and when the temperature is equilibrated, run NPT.
fix 1 all npt 500.0 500.0 100 xyz 0 0 1000
What at effect of pressure in Z direction? This might have an effect.
I hope this helps?