Just thought I'd post this unless someone else is having the same issue.
Its occurring with our older Dell 9010 OptiPlex machines. They have a SSD drive, LITEONIL LCT-128M3S. Nearly 4 years old and been very reliable and fast.

So sometimes (10%) when I image a workstation that has been giving problems, the image takes about 20-to-never mins to download. Normally takes about 12 minutes. Reimaging is slightly faster but after the system is booted up and runs the sysprep stuff etc etc the box is still not a happy budgie. A fault in the image/download maybe, but other machines work fine.

So out of desperation, I deleted the two Win7 partitions from the SSD using 'img'. Then reimage and its super fast (8 mins) and sysprep etc runs fine and in the end, its a happy budgie.
I hope this is not some sort of wearing out failure with SSD partition information.