I have a 1999.5 Ford f-250 7.3 with 405,000 miles on it. It's been a very dependable truck but lately I've had an odd problem. On a 10hr drive, at about hour 5, I had an issue. At a speed around 70mph the truck would start accelerating by itself for a half second and then go back to normal and when I say accelerate I mean that it suddenly would pull (accelerate) harder than if I had put the pedal to the floor on the freeway. It wouldn't pull a lot harder but cirtainly noticable more so. I found this odd but there were no check engine lights or stored codes so I kept going. A few miles later it did just the opposite and "bucked" as some people are calling it. That is that it felt the truck was tried to jolt to a stop only for an instant and then returned to normal. After pulling over and checking a few things, I continued on and after experiencing both of these things a few more times I slowed down to 55 and drove all the way home with only one or two more instances (far less of a frequency than when I was going 70). I've looked on forums to find that a lot of people have had the "bucking" issues but not the random hard acceleration. The people with the bucking issue seem to point to the CPS, ICR, or ICP. I'm leaning towards ICR or icp since it seems like it could be an injector pressure thing. I'm confident the two issues I have explained are related as I have never experienced either issue before. The ICP and icr are relatively expensive oem parts so I would rather not blindly replace both of them because money is a little tight right now. Just looking for someone with some insight or maybe a similar problem

SIDE NOTE: The drain line connected to the back of the fuel bowl at the petcock was leaking a noticable about of fuel when I pulled over to check on the truck mid trip. I replaced this, and every seal in the fuel bowl using a kit from DieselOrings.com because it seemed like at 400,000 miles they may be a little worn. However, I don't think a leaky fuel bowl would cause the random acceleration at freeway speeds so I don't expect this to fix the issue.

 

You can do a few free things to troubleshoot. Unplug the ICP, if it runs normal check the plug for oil. If its oily the seal inside the sensor is bad and the ICP needs replaced. Unplug the fuel bowl heater. If it runs better leave it unplugged. Check the tinman nut on the IPR, if its loose tighten it.

Beyond that you'll want a scanner to run ForScan and log a couple engine paramters

 

Not sure what tranny you have but, my l99 zf6 has done that for 7yrs+...i just roll with it. Some mornings/days, its like it found my stash and wants to do wheelies..haha.
ive looked at sensors, cleaned the TPS...etc, nothing helped. Somewhat chaulked it up to either different type of D2 or an injector sticking....but, to me, i summed it up that the truck found a 9th piston