The two examples you provided for "sometimes" are both OK and they mean exactly the same thing.
The case for "however"
It would be very useful for you to provide a context of using "However". That way, the answer can be more helpful to you.
Usually I use "however" according to the following pattern:
statement1 however statement2
They can be in the same sentence (with "however" in the middle), or in consecutive sentences (with "however" at the beginning of the second sentence).
The meaning of the construct is: statement1 is correct, but statement2 brings a new point of view which is not entirely compatible with statement1.
I like to work; however, I like even more to relax.
Jane is very experienced. However, we cannot hire her because we require a different skill set.