Sometimes the party castbars will show in raid, in the position they would normally be at in a party, but it's not at all consistent.

I'm using Blizzard Raid Frames, and can't see to find a pattern for when it happens.

u mean they attach to the raid frames 1-5? That's intended.

I'll try to take a screen shot next time it happens... it's kinda hard to explain :)
They overlap the raid frames, making it difficult to see player health and such.

As of v1.3.6 party castbars will now be disabled when in groups larger than 5. Guessing this should fix it.