I have a 1991 ford E-250 when i step on the gas after 15 miles an hr it shakes

it sounds like a misfire under load when warm. could be alot of things. plugs, wires,cap,rotor and combustion issues. if its intermitent probably a plug or wire breaking down.

The other respondent, Global Helper is correct. It can have many root causes. A failed head gasket is evident throughout the entire operating range not just as you describe, so I would rule out the head gasket for now. Fuel delivery is constant at all rpm ranges, but the pressure varies with throttle vacuum. See if you have enough fuel system pressure throughout the RPM range, check for open vacuum hoses, replace all if original or show signs of failure( too soft when squeezed). Vacuum and pressure hoses have opposite functions and are not the same. Do not substitute. You might need to scan it for codes. There was a specific tester called a Star Tester for your year model which incorporated what was called an EEC III system( 3rd Generation Electronic Engine Controls). This operation requires that specific scanner to diagnose the system if your Check Engine/Service Engine Soon, otherwise known as the MIL( malfunction indicator lamp)is illuminated. Often the TPS( Throttle Position Switch) fails at the position you mention because most operation takes place within the RPM range that correlates to that portion of the throttle position switch range. deposits. Why it happens is not necessary to discuss here and very lengthy. This is just the introduction to systems diagnosis. I am a former FoMoCo Warranty Specialist, Shop Foreman, Master Tech fully certified in all technical areas of Ford vehicles service and operations. A dirty throttle body is something that represents a fleetwide problem, and in the absence of a scanner is often confused as TPS failure. Find a can of Berryman's Carb Cleaner and an old toothbrush. Have someone floor the accelerator pedal while you spray cleaner into the trottle body, then brush it all around and spray the remaining after engine starts to flush it out. This is a required maintenance step. Respond after this is has been completed.