VAPOR LOCK

Every engine is desighned to operate at a certain tempature,most commonly 190F. The rise in temp you are seeing is the result of restricted radiator,worn water pump impeller,poor or non-operation of coolant fan and/or any number of things. If you are not into this type thing,better get somone who is before engine is damaged. Vapor loc is somthing I hadn't heard for many years but since you mentioned it,I don't think that is the problem. The F series trucks have the fuel pump at the tank and I expect your E is also. Starting with simple things,ckeck for bugs or grass on or between radiator and ac condenser. Look inside at passages in radiator if possible. There should be no accumilation what so ever. If fan is electric,it should be running full speed when temp rises and continue long after pulling to roadside. Raise the hood and look next time this happens. If fan is belt driven,after engine warms to operating temp,kill engine and attempt to spin fan by hand. If the fan freewheels when you give it a spin,the clutch is defective. Is there a shroud in place around fan? That's only a start but maybe it will help.