I think I found a bug dealing with how the platform behavior handles slopes. I'm trying to make the player slow down when climbing a slope and slide down when coming down a slope. The method I'm using works rather well when the player is moving towards the left but when the player is moving towards the right he get's stuck and behaves strangely. The following CAPX should illustrate the problem:
I tried it on Chome, Firefox and IE with similar results. (and Firefox behaves even weirder with the player sprite moving very strangely)
Also, I saw this tread: http://www.scirra.com/forum/jump-thru-platform-positive-x-movement_topic53067.html
...and used the updated platform runtime.js with similar results.
Here are my specs:
Windows Vista Home SP2
GEForce GTX 560