Sometimes when I drive over 50mph, the engine.

How long since overall tune-up? Check for applicable trouble codes. To check for trouble codes, yourself, at the least, you need an obd2 code reader, your local autostore should have one. Some of the national brand autostores will check codes for free.
Have you ever changed the fuel filter and checked fuel pressure? Don't take any fuel lines loose unless your sure there is no pressure on system.