I am sure this is not the case because RPLXi's are awsome rods ( at least the ones i have cast were) But rod design can create this problem.. Over the last few years I have watched my boss design many rods and have seem him work this problem out of many of them. Most recently was with a 10 ft 6wt..
He designed it and it cast awsome great at all distances and very authoritative and tight looped but on ever cast there was a wave that traveled down the loop. It's called a "sign wave" don't know why but thats what it's called
.. anyway so my boss said he knew how to get rid of it so he designed a new part and we made it and when we cast it the sign wave was completely gone. I never would have thought rod design was so important but it sure can be..
anyway I know this doesn't help but i thought it was interesting.. I would guess that somewhere in your forward stroke you are shocking the rod and causing the wave... try to watch you back cast and see if it's occuring there also,,,