Hi Cleo,
make sure you have implemented all the notes/bug fixes from the following 3 'Composite SAP Notes' on your BW Landscape, including any manual steps...
1827854 - Enqueue/Lock during parallel data loads
2096765 - Composite note for hanging Infopackage processes in Process Chains
2093582 - Composite note for hanging DTP processes in Process Chains
This will significantly increase the performance of your process chains and also prevent most of the bugs associated with the failures. You can see which bug-fixes are relevant to your SP Level also.
Please give points if this resolves your problems once you have completed your analysis via transaction ST13,
thanks,
Colm