Oscillation consternation

[m-theory]

I'm stuck in oscillation hell.  I've got two completely different circuits that worked when built initially, but upon being boxes up, have developed uncontrollable oscillation screeching. 

One is a vero-built Zendrive that worked perfectly in a different box for over a year.  The moment I stuffed it into a different box, it developed an oscillation that I cannot seem to nail down.

The other is a modulus magnus, which worked perfectly until I mounted it in a box, and again, has since developed an uncontrollable, continuously howling oscillation. 

I get no noticeable changes in the screeching on either circuit, when moving wires in any way, nor when I touch the boards or components.  It's as if both of them were successful "screech" builds. 

I know that I've gotta get some measurements and get on with a logical debugging effort, but I thought I'd just toss this out, in the remote chance that somebody might be able to at least tell me where I could begin looking for something like this.  It's just bizarre to me, that both would work fine out of the box (and one of the for a year in a different box), and then go berzerk all of the sudden, when boxed up.  All I did in both cases was mount the pots, jacks, etc., into a box. 

[earthtonesaudio]

Input/output jacks not grounded will often cause screeching.

[m-theory]

I'll double check both boxes, but unless something broke loose, I had input and output jacks grounded.  Thanks.  Any other possible causes?