No clue on the intermittent thing, but here's a plug for Aquapel. It's like RainX on steroids. 1 application for 6 months (though, in reality, mine's still going strong almost a year later), sheds water like it's nobodys business. It was developed by PPG (the glass people), and apparently Bonds to the class instead of just coating it (al-la RainX). I can't remember the last time I used wipers.... oh, wait.. I can. It was to clear the road spray after it'd quit raining (not enough moisture to bead up and run off).

At about $6/application (I got mine on eBay), it's going to be a LOT cheaper in the short term than intermittants.

Anyway, just a thought that might help your issue.