What is causing surge and rough idle on my 1996 ford aerostar xlt?

Answer 1

the idler pully may be worn meaning the bearings inside the pully

second possibility the belt may be misaligned or worn

improper tension on the belt around all the pullies

check for poor battery connections like the ground

check for proper exhaust. check air filters. check fuel filters

check carburetor for sticking and or clean. use quality gas

check for broken spark plugs or crack wires and inspect rotor cap

check sensors

best of luck...