Dutchy

I have competed in 5 races this year and during every race my quads have had a spasm/cramp that feels like there is a balloon inflating under my skin. All the quad muscles feel like they are ready to burst. It is a bizarre feeling, and only happens when I stand on the pedals.
It never happens in the first stage of our races, only on the second stage after our halfway break. I drink one bottle of powerade per hour and have a powerbar and powergel at half time.
It only happens during races, not when I go for hard solo rides. I think it has something to do with lactic acid as my heart rate was 178ave for an hour for stage1.
10 minutes into stage 2 and I have trouble standing. Could it be that my legs are cooling down and then in shock for stage2.
Maybe I just need more race experience.

Does anyone else get similar feelings in their legs? and is it a muscle spasm or cramp?