I'm having the same issue but with a 6* GK aged 20. His form has been (4-6-5-5-5), gone as low as 3 but no higher than 6 in form. I bought 2 5* keepers at the start of the season and this one has trained very fast but his form is poor. Yet I have a 5* keeper 5M more expensive than the 6* with much better form (7-6-7-7-8).

Question is, should I sell the 6* keeper with poor form and play my 5* keeper with better form? Or doesn't it really matter?