The problem is indeed not on every submission; I would suspect perhaps there's a data-specific issue, especially as some of the field validation is pretty weak at present. There is therefore some investigation to do but the good news is that it should be possible to put some debugging info to assist in the diagnosis.
If accepted, I'll fix the reported issue and also suggest a number of other improvements that could help avoid future issues.
Obviously $20 is a very small budget to fix your site; a more realistic budget would get you many more quotes and a more thorough solution.
Regards,
Derek Pattenson