My Ford f150 doesn't have the power at highway speeds and won't stay at highway speeds

Twenty-seven year old vehicle. Begin with the easy stuff. Check the plugs, plug wires, etc for proper operation. Changed the fuel filter recently? Check the pressure of the fuel pump. Do checking first before throwing parts at it in hopes of getting it right. How many miles on the vehicle?

If u don't anything after u check the basics, check for a plugged catalytic

Could also be dirty injectors or a bad coil or p/u coil inside the dist. Can you give a lil bit more info on just excatly what its doing back fireing, popping under a load sluggish in the low to mid or mid to high rpm range? The more info you can give the better we can help