I figured that I needed to start from scratch on this.
I downloaded the 3.8 version of ALL libraries for jqGrid (to make sure I had the latest versions), and downloaded the demo examples and did an exerpt from the BASIC navagation grid demo.
I have downloaded the latest version of jqGrid 3.7.2 , added them to my project, cleared the cache and javascript cache and I am still getting this problem with 'clearGridData'.