Yes i had this problem. One of my boys is quite wide and when jumping i found the saddle slipping sometimes. The saddle fits him fine and once i changed to a non elastic girth it got much better. last week i took him to a ODE and during the SJ my saddle started slipping and i realised i had the elastic girth on!
yes but they are much nicer, they cost about £25 and if you machine wash them be careful 30degrees only, mine has slowly shrunk a little more every time i have washed it.
i was worried the strings would pinch etc but it doesnt seem to and my pony is fully clipped and has had no objections to it
If the elastic inserts are on both sides rather than just the near side as is normal then that should eliminate the slipping. I'd recommend the Professionals Choice girths.