Ruined lawn with too much grass seed?

I think I may have used too much grass seed when sowing my new lawn from scratch. The seed was put down in April and the lawn quickly grew to become thick and lush. However, over the last couple of months it has started going very brown and bare in areas. The garden is very sheltered and we've had plenty of rain so I don't think it's due to being too dry. I've read that if you put down too much seed the roots become too competitive and bits die off. Frustratingly, I can't find any information on how to resolve this. I know that raking away the dead grass and using a fork to improve drainage is always a good idea (particularly since we've got heavy, clay soil), but is there anything else I can do?

Honestly-you cant overseed grass-that is unlikely to be the cause of bits dying off-there could be any number of factors

Do you have any pets?or nightime visitors that you are aware of?

Have you used any feed,weedkillers,or anything like that?

Alice, at this time of the year chafer grubs and leatherjackets can devastate lawns by eating the grass roots. You can get a lawn grub killer to control these pests. If you cover a small area of grass with sacking or plastic overnight and remove next day you may see these grubs.
Don't rake your grass too fiercely either...I wouldn't start raking until next year anyway.
Did you apply a late summer feed to your lawn? If so, this may have caused a problem.
Re overseeding......it depends. You can overdo it but I think the grubs In the lawn are the culprits