When I place to much pressure on my bike pedals they skip.?

[Deadeye E]

When I go down hill they're fine because I place hardly any pressure on them at all, but when going up hill or on a level surface (on a level surface they don't skip as much) it's a nightmare. I have to put a lot of pressure on the peddles and they skip and a few times while I'm going up hill and I'm standing up and peddling they skip and I loose balance and fall. But yeah... going up hill they seem to skip like every half a second and it's almost unbearable. Any help would be awesome. Thanks a ton =D