Well there has to be a reason for the partner to behave in this way.
If there is, you must talk it out and resolve your issues once and for all. Marriage is a very holy knot and trust plays a very important role in it. If need be you must also seek some counselling to help you even further. All your partner is needing is that assurance from you so that she is certain of this relation.
If not, you further need to evaluate this problem. Looks like s/he is suffering from uncertainty. If it's mild, simple counselling -may also take a few sessions- should do.
I find it wise to mention that, during this period it maybe hard for the other partner to tolerate this but s/he should understand that the other half is needing a little more care and love.