Open chassis sitting on speaker cabinet

[6v6Pin1]

I built a 5e3 kit and noticed something odd recently.

When I place the chassis directly on the speaker cabinet (transformers up), the amp is unstable. When I bump it, it will start motor boating. I bump it again and it will stop. When I place the amp on a chair a foot away from the speaker cabinet, I don't get any of the instability.

Is this normal behavior because I don't have the chassis in a proper (shielded?) cabinet? Or do I have an issue that I need to fix?

Thank you!

[shooter]

I have an issue that I need to fix?

Yes you do   I would look for a wire that you have a good mechanical crimp but you forgot to solder, a cold solder, a "hair" from stranded wire, etc. 
Use a chopstick to "drum" on wires, tube sockets, etc,  it should be an easy find

[sluckey]

I've never seen a 'motor boating' problem with this. Check the stuff shooter suggested.

It's usually just a feedback or noise problem. But, you need a shield on the bottom (open side) of the chassis. This can be a metal plate screwed to the chassis, or it may be a metal sheet, foil, or screen attached to the head cab that will make good electrical contact to the chassis when the chassis is bolted into the head cab. Same applies for a combo style cab. This would be even more of an issue with high gain amps.

I haven't had any issues in my shop with placing open chassis on top of speaker cabs, but I've never really cranked the amps. I also don't have any high gain amps.

[6v6Pin1]

When I was drumming with the chopstick I noticed the last filter capacitor was very sensitive. It wasn't visibly disconnected but I reflowed the solder joints on that cap and the problem went away. Now I can crank it up while it's sitting on the speaker cabinet with no problems.

Thanks for helping me out!