Miilermatic 185......

Ok lets stat off with the simplest things first and we can go from there.
When you say it is back peddling are you talking about when you are trying to weld it seems like it is burning back to the tip?
If so look at two things. First one is the tension on the spool itself, if it is too loose when the spool spins around some excess wire will drop and then when the slack is taken up it will slow the feed down. Think of this like towing a car with a chain. when you keep the tension snug it is an easy pull but when you have a person in the tow car that doesn't know how to do that it gets pretty jerky.
Second, I know you said you replaced the liner recently but do this, Pull the liner back out of the gun and really look at it. The slightest kink/bend in the liner will cause this issue in a big way and it is very easy to do when pushing the new liner in the gun. If you lay it on the ground and see any kink in it at all replace it. They do make different size liners for guns and if you do have the 023 liner in it a slight kink is even more noticeable than if you were running an 035 liner in it. It doesn't hurt to run a larger liner in a gun as long as you are running the correct size tip. In my smaller rental welders they all have 045 liners in them that way when i send them out a customer can run 023 to 045 in them with the correct tip and diver roll without issue. In my larger wire feeders they all have 3/32 liners in them so they have that versatility
98% of the service calls I go on for feed issues are taken care of by these two things.