Forms apparently not processing

We have what seems a fairly common problem, discussed in various threads.

Users report the page sometimes says it is processing forever on submit (text on our site actually set to “Please wait.”) Sometimes the form actually seems to be processed, though the user is not so informed. Sometimes it is not processed.

It is hard to test since it is an intermittent problem. I have looked for patterns of usage – device, OS, browser – but nothing seems determinant. The solutions proposed in other threads in these forums seem contradictory and unhelpful – roll back to version x of the plugin; no, roll back to version y; no, the latest version solves it.

I submitted a ticket to the official support about a week ago but have yet to see any response.

It seems time to look for another forms plugin. Suggestions welcome.

The page I need help with: [log in to see the link]