My 85 Gmc Sierra classic 1500 has the 5.7 v8 and manual tranny. Whenever I drive it when Its not warmed up all the way it will surge or shudder at 35 miles an hour and sometimes around 45 a little but any other speed it wont. Doesn't matter how hard or easy I am on the gas. It's not a fuel pump or filter issue I'm pretty sure.