12c query performance slow

Hello team,

I am facing one problem, one of my queries was running fine, but all of sudden it is taking time, after some time (10 mins) I rerun the same query it was executing in the same time frame. I am not getting why the query is misbehaving.

1) indexes/table stats are up to date.

2) execution plan is not changing.

3) there is no change is sql.

4) DB version, 12.1

thanks in Adv.

thanks,

Javed