99 ford e350

i have a 99 ford e350 5.4 gas eng

when driving 35 to 40 eng cuts in & out

loss of power eng light not on?

at idle runs ok.

What kind of maintenance has been done in the last 10 years? How old are the spark plugs and wires, fuel filter air filter, transmission fluid?

Get all of that stuff done that should have been done. You need to do that anyway so lets start there and make the troubleshooting easier by knowing those items are not the problem.

Cuts in and out. Please explain exactly what happens.