High SWR Alarm on PGXL - only on CW

I'm getting the HIGH SWR alarm on my PGXL, but only in the CW mode. Alarm kicks in on the initial dit or dah, but not every transmission. No problems on FT8 or on SSB though, admittedly, I haven't operated much SSB with it. I've connected the amp directly to a dummy load and get the same results on CW. I've had the amp since August but only recently started doing CW with it, so this may have been a problem from the get-go. Ideas?