I have had the same type of problem that you have indicated. Basically, I apply a force and once in a while (apparently at random) it "explodes" and applies something like 10x the force that is supposed to be applied.

I don't understand why adding forces is not framerate independent now.

Using:

time_smooth = 0; fps_lock=on;
ph_fps_max_lock= fps_max;

at the beginning of the main function causes issues for me as well. It causes huge slowdown on the rate of things falling for me also.

It is disapppointing is that the physics engine was released nearly two years ago now we are still struggling with several unreliable and/or unfinished functions. The good news is that the physics engine is slowly approaching what it should have been on release.

Jason