Hi,

Which power supply do you have? 

Here are a few things to consider. 

1. You can go for Brake sweep. For sweeps it is not necessary that the RPM follows a certain RPM curve, it just needs to be a smooth sweep through the RPM range. Brake sweep is by nature stable. There is a new (beta) version available now it has a few improvements to the brake sweep function (sweep up and down are supported, plus static brake percentage start),  http://www.yourdyno.com/YourDynoInstaller_3.01.69.msi.
2. Also for RPM sweep, do not worry too much that the RPM follows exactly the curve. The readings come from the actual RPM and brake force (plus acceleration), not from the RPM sweep curve
3. It looks like your engine/brake/power supply combo has different sensitivity at different RPMs. This is not uncommon. The engine is probably much less punchy at 1200 RPM compared to when at full power. If you want to stay with RPM sweep, you have two choices: You either go for the lowest setting necessary for the whole sweep to be stable. Or you can use the Advanced PID setup for this. If you have a set of parameters that work at low RPM and a different set that work on some higher RPM, then  you can calculate what the multiplication factor needs to be. You can make a curve of multiplication factors (but dont make it too complicated). Start for example at 1200 RPM and set the optimum parameters for this RPM and the multiplication factor there to 1. Then for some higher RPM set the multiplication factor to 0.5 or 0.25 or whatever you need. 
4. A sweep will typically need lower PID parameters than a steady state. So even if Kp of 2 makes it stable at 1200 RPM, this starting point is probably too high for a sweep. This is another reason why a brake sweep is simple. You just need the parameters to be right for that one static starting RPM.

Cheers,
Jostein