I have a Rapido LRC locomotive I'm trying to get running smoothly. 

Several times I've done what Rapido calls the "awesome slow speed thingy" in the manual (setting CV54=0, and the pushing F1). After the most recent run of this, the LRC has these CV readings:

CV52=18

CV53=86

CV54=49

CV55=18

CV56=255

But the problem—after each time I run this procedure—is that the loco surges/pulses at settings from about 25% to 75% throttle. At slow speeds and at high speeds it runs very nicely. But in the middle throttle range, there is a noticeable pulsing/jerking; it does not run smoothly.

I'm running on a basic Digitrax Zephyr system. I'm a newbie to DCC, so I've had to do a lot of googling on this subject. I've found suggestions and tried adjusting some of the different CV settings, but have not found a "sweet spot". 

Can anyone provide some advice or assistance on this? Or point me in the right direction?

Before doing the LRC, I ran the "slow speed thingy" on my Rapido F40PH, and it runs beautifully after doing so.