Has herpes been ruled out? A swab can be done of one of the lesions and sent off for a culture. If it turns out to be herpes, then supressive therapy with Valtrex (one 500mg tab daily) would help.

A B12 or iron deficiencies can cause oral ulcers. A blood test would help diagnose this.

Some people are sensitive to sodium lauryl sulfate, found in some toothpastes.

Just some ideas ...