Thank you for all suggestions. 1st, I have firmware 014. 2nd, the dummy load is ruled out because the same thing happens through a wire dipole. Nano VNA used to check SWR which is well below 2 across 20m. I have now observed 2 different issues - sometimes the lock up happens as previously described, sometimes I still hear sidetone but the s-meter and DWR meter will stop showing any output. In reply as to why I wasn't using practice mode - I wanted to see if the s meter was working. And yes, the letter 'S' appears when lock up happens.
