Larry,
You need to pull the last of the wire out after it runs out of the feed rollers. If you don't, you stand a good chance of the new wire to jamming inside the liner. On the big industrial machines I used to run, guys would sometimes pull it through just enough to get the new wire started, thinking they might save some time or just forgetting to finish pulling it out and then spend the next 20 mins taking the liner off the wire feed unit and unjamming.
If it starts feeding good it should end feeding good as a rule. Are you exceeding the duty cycle by the end of the weld? If the rollers are getting dirty, the wire is where its coming from, most likely. Is the spool inside the unit or outside? One way to keep the wire "dusted" off is to take some folded up paper towel and clip it around the wire before the inlet tube to the rollers. Just get one of those big clamp type paper clips from the office supply and let it hang on the wire. This wipes the wire as it comes off the spool. Make sure it doesn't ground out on the wire anywhere. Change the paper towel periodically or the wire will cut through the paper and the clip could ground out the wire and then you have a real mess of spaghetti
Hope this helps