Tip dip will help a little. I've never used a SIP, but TIG Paul's explanation is the most generically likely. Doesn't need a lot of spatter. Ceramic shrouds a la TIG might help, but I don't think anyone makes them for MIGs.
I had same problem with my portamig and there was no visible spatter,Steve advised and supplied new diffuser for the torch.Problem solved,thanks Steve.
Regards
Martin.