Filament skipping

Filament skipping

I am hoping someone can help point me in the right direction.  I am running a Railcore II ZL with core xy setup using a Bondtech extruder and a Mosquito Magnum hotend.  I am running a 0.4 mm Tungsten Carbide nozzle (love these nozzles by the way, better thermal properties than brass and harder than steel!) I am printing PETG with nozzle temps at 250 and bed temp at 90.  I usually don't get the filament skipping until the second layer.  I am not sure what to adjust to help.  I know the second layer is printing faster and slowing down the print will usually help.  I have heated the nozzle up to 260 as well and this also helped a little.  The funny thing is it doesn't always skip, it varies from print to print.  It is definitely the extruder slipping though as I can here slip almost sounds like it is retracting but it isn't. I have also tried various different tensions on the bondtech but that didn't seem to help and may make it worse.  The only thing that has helped is slowing the print down.  With the magnum I was expecting to print fast and the printer can definitely handle the extra speed.  Can the filament and the hotend just not keep up?  Is it possible to be getting heat creep all the way up into the extruder?  There is about a 60mm ptfe tube between the bondtech and the mosquito.  My speed for infill is about 140mm/s when skipping and have to slow to around 100mm/s to get skipping/slipping to stop.  Any help here would be great. I am thinking about trying to find another part to allow me to add a bigger hotend cooling fan, but not sure if heat creep is the issue or how to tell for sure.  I am also in the process of enclosing the printer to do materials like nylon and ABS, if it is heat creep this will just make things worse or not function at all so how do I fix that?  Thanks for your help.