99 suburban no start fuel

k so here's the deal... 99 Chevy Suburban 350 5.7 vortec k1500 4x4 about 210,000km (I know....) a few months back it started running a little rough, not accelerating as it should. It would only allow me to get to about 80km/h that gradually started to decrease to about 40km/h up until it would...