Up until 2006 I rode Mountain bikes and it was pretty aggressive stuff aswell. In 2001 I had a Prostate problem and after surgery I had a saddle problem. This was down to the Surgery and a cure that worked for a while was a saddle with a cutout in it. Unfortunately- the surgery continued to throw a problem at me and it was severe butt Ache. But that cutout certainly did assist with the pain. So first suggestion is to look at the saddle. Is it far enough forward? 68 deg sounds like a very slack seatpost angle to me and you may have to bring the saddle forward to stop sliding forward. My MTB has a seatpost angle of 73 and the road bikes are 74.

And secondly- in 2006 I bought a road bike. Instant cure. May have been co-incidence but doubt it. I have thought about this and on a road bike the torso is more stretched out and the butt is in a different position to on a mountain bike. That completely different seating position kept me cycling and it seems all wrong

Have a suggestion and that is to go out on a ride and get Butt Ache but finish up at your LBS. Sit on a road bike that is approximately set up for you and see if you can feel the saddle. Could be like me that the different seating position will work. With the butt ache still there- you will know if it will work.