I posted something for you to try but I guess it didn't take. Oh well here it is again.
Start with your heels as close as possible to the foul line and then do your approach towards the ball return, slide and everything. Where ever you stop either right there is where you should start or you can move another 4 to 6 inches back (this is something I do as a precaution, more of a just in case thing).
Granted if you are starting somewhere different from where you usually start you will probably have to adjust because the angle into the pocket will be different if you are further back or closer to the line.
Also might be a good idea to check your
shoes. That could also help.
The last thing I could suggest is to maybe shorten the first step into a slide. It might help. I have a 5-step and that's what I use.
Are you sliding over the foul line or are you stumbling and stepping over it? I forgot to ask. If it is the first than the little technique I mentioned above should take care of the problem.