Problems with Lincoln Redi-Mig 455

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Greetings fellow welding enthusiasts.I'm having this problem that has followed me ever since I started using MIG about 18 years or so and I have decided to try and get to the bottom of it.I am currently using a Lincoln Electric Redi-Mig 455 with 4D remote. With Lincoln Ultra mag S6 Precision Level Layer wound Mig Wire 1.2mmWe are doing a Job for Sydney rail. And requires full strength butt welds on about 50% of the job.In my experience it is easier to turn the power to around C d and wire speed up to just after 9.This gives the weld a pooling quality which finds its own level when fusing to the back of the root run.The problem I have is that at this setting, after about 2.5 - 3 metres of free flowing weld, the wire starts to stutter and burn back rapidly leaving me with an unattractive and inferior weld. Now you would think that the machine would be able to handle the power settings. They are nowhere near the highest.I have tried changing the liner, changing the drive rolls from the normal ones to the ones for core wire that have indentations that grip the wire as it feeds, pulling the cable straight so there are no kinks in it, changing the contact tip from a 1.2 to a 1.6 (My theory being there will be more room for the wire to flow through the hole even if the tip expands in the heat), renewing the earth cable and earth connector and clamping the earth directly to the job, using nozzle dip after every 10 - 15 welds. The only thing that seems to work is when I change the contact tip for a new one, but this only lasts for around 3 metres or so then it starts stuttering again.Has anyone out there in welding land ever came across this problem and know how to fix it?Cheers in advanceThe Cheeky Monkey.