1990 GMC Sierra Problem fixed. It was the fuel filter.

My 1990 GMC truck has a 350 in it. It runs good but when you are moving and give more than half throttle like to go up a hill it just dogs down. It almost sounds like a vehichle does when the fuel pump is going out but it works fine if you don't give it more than half throttle. What would cause this? Sorry i can't donate untill next week. Thank you