Hello Diana,
the explanation why the workprocess re-start with the message "WP has reached abap/heaplimit" could be found in SAP Note "1571845 - Error WP has reached abap/heaplimit- What does it mean?"
System resource usage (both memory and CPU) has increased with 7.4x kernel, so you could increase this parameter (double it) and see if the situation improves.
kind regards,
Mercedes