Was this the reason for bonnet strap maybe?
Are your corded pads making contact with the bonnet halves, these would at some resistance to sliding?
When I had my Roadster, the bonnet catch latches (i.e. the fixing on the bonnet) would slide forward at speed. There were witness marks on the grease on the fixed part of the latch... (from a nearly central position to off the catch altogether) furthermore, the bonnet would actually start to lift at speed.
I drove it through France without the bonnet strap (mistake) and had to keep my speed down as I could clearly see the bonnet lifting. When I ordered my +8 I specified a bonnet strap for this reason (as well as liking the look).
