1989 f250 351 5.8 l

Hi I have a 1989 f250 5.8 truck that won't
low idle without my foot on the pedal. Fuel
pump and lines are fine checked. Has
brand new distributor and cap. Plugs
wires. Etc sensor, map sensor, Egr sensor,
tps sensor, and ignition module. Timing is
right and runs great with your foot on the
pedal. I also chased down and cleaned all
but one ground. It is the main one from the
motor to the fire wall. Could this be a
computer problem. Or there is one ground
I have not touched from the motor to the
fire wall. The motor side hook up ,bolts to
the bottom side of the fuel injection
manifold and used to have a connector rod
that held a bracket connected to the
exhuast manifold but the piece has rotted
into two . The connector piece also was a
support for the  smog control. Would that
cause voltage spike even though it is still
grounded to the fire wall??
Looking for any suggestions on what else it
could be. Also the truck computer codes
that are flashing are 21. 34.  84 then the
second set of codes are 34. 51. 53. Any
help once again would be appreciated