You experience issues with the integrated enqueue work process (ENQ) as part of the classical ABAP central instance.
You are using release 7.4 with classical central instance setup with enqueue work process.
All 7.4 kernel versions of the SAP Application Server ABAP are affected. Symptoms include a high number of enqueue requests in the request queue, work processes hanging while waiting for an answer from the enqueue work process and high memory consumption of the enqueue work process.
SAP strongly recommends the migration to standalone enqueue as described in this note for all customers currently using the integrated enqueue work process.
Check SAP Note 2013043 – Performance Problems with Enqueue Work Process