Speed workouts and stress fractures

I gave myself a tibia stress fracture 11 years ago while ramping up too soon when I first started running. I re-injured it (same place) 3.5 years ago. Since then, I think I've figured out what the culprits are (increasing mileage too fast, vitamin d deficiency, not enough s&c). Since the last injury, I've run my first 50k, 50 miler, and 100k, and eyeing my first 100 miler either later this year or beginning of next.

However, there's one really stupid issue that keeps bugging me. I occasionally feel a slight twinge from the old fracture site - seemingly random, and usually goes away after a day or so. But there's one particular exercise that really seems to irritate it: speed workouts. Every time I try to do interval training, my leg goes crazy. I always back off and go back to steady pace running, which doesn't give me any issues. But I don't seem to be able to do anything to increase my mile times, which is really frustrating, as I'd like to complete my first sub 2 hr half later this year (current best is just over 2 hours, normally 2:05 to 2:15).

Does this happen to anyone else?