"sometimes must" is a correct and usable phrase in written English.
It is often used as a phrase to express necessity. For example, "Sometimes we must make difficult decisions in order to do what is right."
He sometimes must search his internal archives to recall names.
New activities sometimes must be tweaked after introduction.
The people who service tanks like this sometimes must go to extraordinary lengths.
Now, travelers sometimes must wait up to two hours for a flight.
"When ideas cross into action, though, we sometimes must take a closer look".
Firefighters use a large tanker truck and sometimes must draft water from local ponds.
"You know how editors set artificial deadlines," Mr. Alexander said, sounding diplomatic, as editors sometimes must.
Women with recurrent urinary tract infections sometimes must take low-dose antibiotics for months to control the problem.
Chagas scientists are used to incremental progress, low recognition, and poor funding; they sometimes must scream for attention.
Observant men complain that they sometimes must pray outside the mosques, because so many young people are crowding inside them.
(King's sells local salmon, but the Petersons sometimes must rely on farm raised salmon from Chile or Canada).
