Erratic cruise/speedo problem

Truck is a 2000 Freightliner Classic. It is equipped with a bullydog hdgt so I can see what the ECM is broadcasting. Maximum cruise set speed is 62 mph.

The problem: speed up to max speed, set cruise, it thinks the truck is slowing down and then hammers the fuel until you are about 68-70, then realizes its going too fast, then cuts out. Repeats. The ecm shows the correct speed until power is applied to more than about 50% engine load, the ECM (and speedo) shows that you are slowing down into the 50s, then hammers. If I cruise with my foot, and keep engine load low, the speedo and ecm read closer to what actual speed is. If I let it coast, both ecm and speedo read perfectly.

When I bought this truck, the speedo was seized, that was April 2013. New speedo installed then, has been working perfectly. Appears to be reading the distance correctly for where the ecm tells it to point. Mechanic has looked over this and has replaced trans speed sensor and adjusted the air gap. I have also recently had the alternator replaced, not sure if this makes a difference, as the engine works harder it seems to read more off - could it be there is some kind of bad ground somewhere?

Any ideas ?

 

Well. speedo problem is still here. Yoke nut is tight. Had new speedo head put on when I bought this truck April 2013, it has 172,000 km on it so far. New speed sensor on the transmission and adjusted twice for the spacer (by mechanic) ... when in 18th gear, speedo shows exact speed truck is moving IF no throttle is pressed. Apply throttle to maintain speed and it dips below actual speed (never shows faster) erratically by a few mph ... which throws the cruise off so I haven't been using cruise at all. Cruise works fine if I want to just put on high idle while not moving. However, if I am driving and need to go down to 17th and then mash the throttle, speedo shows about 50% of actual speed .. again, let up on throttle, shows correct speed ... I believe it is also reading the distance traveled less than actual distance moved now also. Today google says I should have drove 964 km, speedo shows 888 ...

I don't even know where else to look at this point ...

 

Thing is - it reads the speed exactly what it is if there is no throttle applied. All the way down to zero if I coast it down. ??

 

Question on that Heavyd,

Any idea on what would cause the speedo to dip much more while the splitter is pushed backward, as in 17th, it seems the more fuel you give it the more it dips, however in 18th with splitter forward, it is only slightly erratic? Same thing happens in 16/15 all the way down, just more noticeable in the bigger gear because of higher speed

Detroit 12.7, ddec4, engine is a 99