88 Jimmy Engine problems PLEASE HLP

I have a 88 full size jimmy with a 5.7l v8 with 97,000 miles. I bought last month. so not sure when any services have been done..But my prob is that when i start from a stop and i give it gas it makes a like clunk sound and hesitates and after a rew times of this it will start to take off. but that's not the only time it will do it.if i've been driving for a bit it will start to do it again..Could it be the timing,fuel pump or what? I'm really lost please help