Not specifically familiar with the construction of the Ideal windlass so the comments below are based on generic, horizontal windlass design.
I wasn't 100% clear on whether you meant the chain is slipping in the gypsy, that is the gypsy keeps turning but the links of chain jump out of the slot in the gypsy, sort of like cogs of a pair of gears jumping out OR is the gypsy and chain staying together, the motor
is turning but the gypsy is not?
If the first then it is an issue of fit or wear with the chain and gypsy. If the later then it is an issue with the clutch
If the gypsy is worn (but you say the chain lies in the gypsy snuggly) then replacing the gypsy may fix it.
Also, is it possible that with the new chain it is not laying well in the chain locker, piling up and not keeping enough weight on the chain to keep the links in place. Does the "slipping" get worse as the anchor
is closer? That would indicate this could be the issue.