Hamstring Tension When Running Intervals

I've been training consecutively for the last 8 months, there hasn't been 2 consecutive days I haven't trained for this time. I've been running at least 4 times a week and one of those was always a speedwork/interval running.

These days I've started to get these weird cramp like tensions in the back of my right leg, hamstring whenever I try to push myself into a 400 meters intervals. I'm not talking about the regular ones you get in the last quarter. Right after the first strides it kicks in, no matter how long I do warm up runs and stretches. I don't have this problem when I'm cycling or swimming. Could the reason be overtraining? I don't feel tired, mentally or physically. I can run for hours if I'm not sprinting. What am I doing wrong? Should I take a break or just skip the sprints? Ironman is 11-12 weeks away, so I'd rather just keep running.

What would you suggest? Is there a special  kind of warm up thing that I should be doing?

Thank you for your answers.