[sdiy] 78xx regulator gives negative output half the time

[Karl Ekdahl]

Hi list

Soooo i stumbled upon something i've never seen before; a 7812 regulator i'm using is *sometimes* latching up at ~-1.2V instead of its supposed ~12V output. 

When i first discovered this, i could overcome it by unplugging the AC to my rectifier and immediately plugging it back in - seemingly it'd un-latch if there was some positive voltage stored in the rectifier caps. 

Another thing i noticed is that if i disconnected the over-voltage protection diodes (going from the output to the input) the circuit would not unlatch whatever i did. 

I've slowly desoldered piece by piece from my circuit board to try and find the problem but to no avail. At the current state, without any ICs in the circuit wse., the psu works fine 90% of the time and latches up the restoring 10%. 

I'm using the most common PSU scheme ever that i've used many times before (www.sdiy.org/knas/temp/psu.jpg) and i have no clue what's going on.

Has anyone seen this before? Any common causes for this problem?

Thanks

Karl