Reverb Gone Wild

I run AVLinux 6.0 and use Ardour 2.8.16 and Ardour 3beta5. I have an issue when placing Calf reverb on a stereo buss. If I quickly initiate the stop/start transport in Ardour(usually via space bar) a "feedback loop" often(but not always) commences which literally is so loud I've had to throw my headphones to the floor. The meter on the verb bus and one (or any) track connected to it via a send show sometimes +400 db and more. Consequently, I've stopped using calf reverb and the problem has since disappeared. It's happened in both Ardour 2 & 3.