I just had similar issues combining DX11.Particles and SplineBuffered. If anyone else has troubles with this in future here's what helped me:
-Note the AsAttributeBuffer nodes have to be the Fixed-Size version.
Otherwise Verlet will not be happy and act as if the spread is being constantly resized.
-As suggested above scaling the splines to 0 at the end of their life is a good way to get rid of them elegantly.
If you want to do constant emission then there are a couple of workarounds.
-You have to keep the particle count maxed as instancenoodles expects fixed spreadcounts. It's possible to do that with constant emission if the particles live long enough..EG If your particlecount is set to 1000, you emit 5 particles a frame and your framerate is 30 fps than you need a lifetime greater than 6.6 seconds. This ensures your particles live long enough that all of them get spawned and when they die they are replaced instantly, keeping the total particle count at max. (To check the number of alive particles create an empty DX11.particles selection node and put it in the particle system. Give it a name. Then use a DX11.particles count node underneath the system and it should give you a total particle count.). You can always arrange to scale them to 0 if they get to a point you don't want to see them anymore.
-To prevent seeing tails when the splines are reset you can either scale from 0 at birth or create a reset buffer for Verlet. This will give a high value when particles are spawned, preventing splines drawing tails back to the emitter location.