Is it common for patellar tendonitis to keep flaring up? it seems like mine does about every 3 months.

Patellar Tendinitis: Yes, stubborn problem. Treatment options include nsaids (oral or topical), ice, brace use (chopat patellar tendon strap), and exercise modification. Specifically, eccentric or negative quad contractions have been shown to be particularly helpful. You could see a physical therapist to help you with all of this as well as try other modalities such as ultrasound.