SQL Server - inconsistent insert speed, fast then very slow -

i'm trying diagnose slow insert thought had fixed problem returning intermittently. it's literally straight insert statement, runs every evening , inserts around 80,000 rows in table 120,000,000. table heavily indexed. insert take under minute , other nights take 15 minutes.

• there no triggers on table.
• i've verified database , log have plenty of free space.
• the primary key identity column

i able reproduce slow run of statement , activity monitor showed following:

status: suspended, wait type: pageiolatch_ex

i'm confused why runs fast nights , slow others. i'm open insights , can provide more detail if necessary.

update - tried various fill factor settings, no change.