I've just been doing some converting of VHS tapes to dvd , I've used -Rs and +Rs .

the +Rs seem fine but all the -Rs have this weird slipping every now and then , they slow down for a second or two .

anyone else notice this , or is it just a bad batch of -R discs I've got .

Can't say I have noticed that. It could just be a bad batch of discs, or the dye used on the discs has changed and your recorder doesn't like the new one, or even that the drive is slowly developing a fault.

I have a couple of DVD recorders with odd foibles. The oldest, a Pioneer only works with DVD-R/RW discs. It's incredibly fussy about modern discs, I can only get it to work with Verbatim media that are fully 1-16x speed certified for -R. My Liteon now works only with DVD+RW, whereas previously it worked with most types and brands, but my Toshiba DVD/VHS Combi seems happy with pretty much any disc I throw at it.