We set up again another stability test (10/9/2013) with Z1 auto shim and the results are encouraging. The lineshape and intensities were very stable that too in the initial part where we saw degradation before without Z1 shim. The lock level also remained steady.
Next course of action: In order to isolate if it is really Z1 shim or magnet pressure instability, we will install the manostat and repeat the stability test. This might help us narrow down the issue further.