I'm having some random bucking issues the last few months and its driving me crazy! I have been searching on here and can't seem to figure out whats wrong with my truck.

The truck was running fine the last few years with no problems at all (well other than my dashboard crumbling) and I bought a motorcycle and parked it mid march. I rode the bike to work so the truck just sat until September. Had a dead battery so I charged it for a few days then it started right up. When I'm driving it will start bucking, backfiring from the intake (sounds like it anyways) at around 60 mph. It would only do this with the O/D on. So I would turn it off when I first start the truck and it would run fine. A few weeks ago it started to do it with the O/D off, and it has started to happen at any speed. It will do do it until I floor it the RPM's will jump and it will go away until I let off. It will do this a few times then it will run fine for a while 10-20 minutes and start all over again.

I've noticed that shortly before it will do it sometimes it will start sucking massive amounts of air into the intake. Like I can hear it inside my apartment.

So here's whats been done to the truck.

at 115k replaced the plenum
at 117k replaced the motor due to blown exhaust valve being misdiagnosed by the dealer as blown rings
Now it has 140k and this is when the problem started.
I've replaced the IAC already about 2 weeks ago with no change.
I'm thinking it may be the upstream O2 sensor or a clogged cat. Does that sound like what I need to try next?

 

Unplug the front O2 sensor, and see if the problem goes away. It its 'better', but still present, physically remove the sensor from the exhaust, and try again.