A few questions ed, if you don't mind.

You've been married 5 years - how long were you together before that?
How long has this once every 4 months pattern been going on?
Is the sex good when you do have it? She says it is painful - what do you do to prevent / minimise that for her? Does she enjoy any sexual contact? Do you enjoy the contact you do have?
Did you ever have frequent (once a day, for example) sex?
Has she had other partners before you?
Do you ever try to have sexual contact without penetration ? (assuming it is the penetration that hurts)
What kind of pain does she complain of?
What size is your cock? Roughly - Are you hung / average?

With a few more specifics I think you'll find a lot of folks here will have a lot of helpful suggestions.