When it comes to your sliding forward, that is probably a problem with the saddle angle (i.e. the nose slopes downward) - I have that issue with the bike I regularly use in spin class, since its saddle needs to be tightened to stop the nose from slipping downward.

As far as the pain.. I'm not really sure, but I have issues with my right hip and get pain in a similar location as well. In my case, it is definitely not muscular - it's either a tendon or a ligament being a bit too tight or loose (my PT never defined which one, or at least I can't remember) and my hip joint will get out of place and need to be "popped" back in. I experience the same pain quite a bit when cycling or when my hip is in a certain position, and I alleviate it by standing and opening/closing my hip angle a bit to "pop" it back to where it should be, alleviating the pain. It can become absolutely excruciating if I cannot adjust my hip angle.