Handwriting and PD.
I've was diagnosed with PD in early 2011. Trouble with handwriting trailing off after a few words was an early problem and symptom. Hopefully now some encouraging words.-
Once my medication was sorted, and that varies considerably from person to person and for all, necessarily needs to be reviewed from time to time, I found that my writing improved considerably. I can't write as quickly as before PD and if I feel trail off, I stop for a few seconds and then continue. In my case I find I have to concentrate on how I'm writing as much as what I'm writing.
My sister has had PD for fifteen years. She goes to calligraphy lessons to keep some writing skill.
I wish you luck and hope things improve; writing wise. You may have to sacrifice your previous writing speed quite a bit.