how to overcome the data loss problem while converting stp to igs or any other format?
Use a direct converter that was specifically written to convert data from one format directly to another.
It is best, when possalble to stay in live file type.
going to a stp file from Catia it is help full to make an allcat part for assembles first as some programs can not handle re opening an assemble saved into a stp file,
also it help to take some time to clean up a file be saving out a stp file.
what kind of data loss are you seeing? is a good place to start?
most cad software cad handle stp and igs with little issue so the use of a third party program / converters is unneeded