I think it's a fairly common problem. I read in some recent Runner's magazine that stomach/bowel issues were really quite important in runners. (September 2006, by Kristin Bjornsen, p. 53)

I also find myself to have much more... hum, active bowels when I run a lot. And I can get a pretty nasty "slushy feeling" sometime that just makes me want to stop. My case fortunately doen't stop me from running.

Maybe if you ran first thing in the morning? I can't run for more than, say, 45 minutes on an empty stomach, but that's a start.

Otherwise I also think there might be something in the lactose. Examine your diet... I NEVER have any milk before I run (although I do have yogourt).

Here are some items taken from the aticle:
- Eliminate dairy 24 hours before running (milk, cheese, ice cream)
- Try acidophilus milk and yogurts with live and active cultures, or replace by soy, rice or almond milks;
- REduce fiber intake (to reduce gas and cramping); replace high-fiber fruits and veggies by low-fiber fruits (ex: cantaloupe, tomatoes)
- Some sweeteners cause GI trouble, stuff ending in -ol. Avoiding gels with fructose as first ingredient.
- Avoiding cruciferous veggies (broccoli, cauliflower, cabbage...).
- ... and beans

Emphasize carbs the night before running.

Be careful with caffeine.

Other things they say: "Easy to take : Sidestep stomach aches with these foods:
- Digestible dairy
- Low-fiber produce
- Refined carbs
- Healing herbs (mint...)
- Root remedy (ginger)."

Good luck. That's terribly annoying. I've found that cutting off milk in the meals immediately before the run solves the problem for me but YMMV...