Forms hang and users cannot access records

We are facing issue in our EBS 12.1.3 applications with database 11.1.0.7.0 where users cannot access record(s). Normally it happens in order organizer form in which when a user tries to open order but form hangs and it continues to be hanged until we kill session(s). Some times users try to book order but again forms hangs and users become unable to book and ultimately we kill session(s) (as last option). It happens randomly and we cannot say with surety that at which order it may occur.

What may be the causes for this form hang and how can we correct this issue?