Recurrent Aphthous Stomatitis: This condition is characterized by the periodic appearance of benign, painful ulcers in the oral cavity, including the tonsils. It's a common condition with a high prevalence, making it the most likely diagnosis for intermittent tonsil ulcers.

Tonsillitis: Bacterial or viral infections can cause inflammation of the tonsils, leading to ulcers. This condition is common and can recur, making it a plausible cause for intermittent tonsil ulcers.
Oral Trauma: Accidental biting of the tonsils or other oral trauma can cause ulcers, which may appear intermittently if the trauma is recurrent.
Nutritional Deficiencies: Deficiencies in vitamins such as B12 or iron can lead to oral ulcers, including those on the tonsils, and may present intermittently.

Oral Cancer: Although rare, oral cancer can present with ulcers on the tonsils. It's crucial to rule out this diagnosis due to its severe implications and the importance of early detection.
Infectious Mononucleosis (Mono): Caused by the Epstein-Barr virus, mono can lead to tonsillar ulcers and is a significant diagnosis not to miss due to its potential complications.
HIV/AIDS: Oral ulcers, including those on the tonsils, can be an early sign of HIV infection. Given the significant implications of this diagnosis, it's essential to consider it.

Behçet's Disease: A rare condition characterized by recurrent oral ulcers, including on the tonsils, along with genital ulcers and ocular inflammation.
Pemphigus Vulgaris: An autoimmune disease that can cause oral ulcers, including on the tonsils, though it's relatively rare.
Crohn's Disease or Ulcerative Colitis: These inflammatory bowel diseases can occasionally cause oral ulcers, including on the tonsils, as an extraintestinal manifestation.