Zf 5hp24 transmission issue

[Themoog]

Alright peeps.
Nearly 3 years ago the transmission on my 01 V8 lost drive so I had it rebuilt by a specialist at a cost of £2000..
All OK apart from a few months ago and again today it would not shift up past 2nd gear in auto mode. OK in manual mode. The first time I didn't think much of it as it was a very short journey down the road to my house. Today I drove it for a mile to a train station then switched off. Upon the restart it worked OK. Drove for 5 miles no problem and I'm currently sitting in it waiting for someone so when I restart and drive off I'll see if it's OK still.
What could cause this? Both times it was from a cold start and it's only done it twice.
Sticky solenoid maybe.. The fluid should be OK as it was refurbished. Ideas?