Wiper Delay Not Consistent?

[GMC D-Max]

One of those annoying, niggling little flaws in my otherwise perfect truck....
When I set my wipers on the SLOWEST delay setting (which is pretty much PERMANENTLY during the winter/spring in western Washington), sometimes it will wipe once every minute, and other times it's like every 5 seconds. It never seems to be consistent, and I don't believe these trucks (GMC 2500HD SLT) are equipped with rain sensing wipers. Any ideas out there? Do I need to be taking my rig back to the dealer for a look-see?

[Smorey]

There was a similar problem with my 96 Burb. I had to replace the intermittant relay box under the firewall.

There was a ton of threads on it in the 6.5 forum. Not sure if the new trucks use the same relay. Piece of cake to do yourself if it is.