Hamstring problems causing weakness in leg when under pressure in a race



I have some slightly strange symptoms that I wonder if anyone could
give advice on.

Basically I last about 20 miles or so, often under quite a bit of
pressure during that time, ie having to work hard, but generally
feeling quite strong, and then my left leg hamstring tightens and gets
very painful (like its full of lactic acid) and that leg loses all
power. Resulting in me not being able to hang in the road race bunch,
which is annoying 'cos I think I actually have a pretty good level of
fitness at the moment - having worked hard all winter.

Interestingly I had it in the middle of a race a couple of weeks ago,
but managed to ride through it and it was fine after that.

When I reduce the intensity (by slowing down - usually off the back of
the bunch) the pain goes away - as though the lactic acid is being
removed, just like normal muscle, but why is my left hamstring
affacted so much earlier than my right leg.

I havent noticed this in training at all, just at the gym and in
races.

I have been getting this at the gym on the exercise bikes also, but
slightly different. It started off with the muscles feeling week and
slightly tight, but I managed to warm it up and stretch it and just
ride through it and it felt fine (maybe that was the wrong thing to
do!!)

I've never had this before, and this is the first year that I've used
the exercise bikes in the gym (as I now work in the city) - so maybe
its them?

Has anyone experienced anything like this, or have any
recommendations? Please.