Random wrong results from expression rule in process model

Hi, we are having a problem executing an expression rule, which contains an a!queryRecordType, where we have:
- 1 grouping field
- 1 measure field
- 4 filters, 

on a batchsize of 500 items.
In detail, every time we try the rule from expression rule the result is correct but when we insert it in a script task from process model it randomly gives wrong results (correct measure corresponding to wrong group fields). A delay of 1 minute after last node has also been inserted in the process  activity.

What could be the problem?

Thanks, 

Francesca