The phrase "frequently occur" is correct and usable in written English.
You can use it to describe something that happens regularly. For example: "Internet outages frequently occur in the summer months."
Bacillus frequently occur in chains.
In the Bible, good and bad fortune frequently occur in seven-year cycles.
Speculative bubbles frequently occur during periods of financial innovation and deregulation.
Delivery delays are often attributed to the parcel carrier, but mishaps frequently occur before an order leaves the warehouse.
These and other attacks frequently occur in public gathering places, such as cafes, markets and other public venues.
However, spring blooms frequently occur.
Spiro heterocycles frequently occur in bioactive molecules.
These systems frequently occur in applications.
Apnea intervals frequently occur in premature infants.
Pressure ulcers frequently occur in hospitalised patients.
Small gasoline spills frequently occur at gasoline dispensing stations.
