weird pitch bend behavior

I have been having the same behavior.

I'm using an M-Audio Keystation 88 connected via USB. I'm at a point now where using Mainstage in a live performance is not possible due to the erratic behavior that comes about.

Basically, I will be in the middle of playing something, and Mainstage registers a pitch bend where the sound comes out an entire whole step below what I'm actually playing on the controller. When you see the pitch bend wheel on Mainstage, it is all the way down. I can click on the pitch bend wheel in the software to get it back to where it has to be, and this will get me back to normal for a while, but eventually, whether it's a few seconds later or a few minutes later, will happen again.

Using the pitch wheel to try to correct the issue is of no use. I have to physically modify the pitch wheel setting on-screen.

I've enabled filtering for the input, but the problem continues. Appears to be software related.

I have also noticed that the problem appears to be limited to splits.

Running on a 2012 MBP 15" with Mountain Lion.

I haven't had good results using USB cables for live performances. I don't know if your pitch bend problem is related to the USB connection, but in live situations, I've come to rely on USB to MIDI cables. the MIDI connection at the keyboard end is rock solid. I use a 3-keyboard setup live with all three connected via USB to MIDI cables with great success.

If you have any other faders, knobs or pedals assigned to pitch bend it cause a conflict. I've had a situation where I opened Logic 9 while Mainstage 2.2 was running and all my patches transposed down a step. I think it was because Mainstage was set at 48k sample rate and Logic's sample rate was 44.1. If you use MIDI Monitor(free from snoize) you can monitor what MIDI signals MS is receiving and sending to determine if it's the keyboard or MS causing the problem.

Same issue here, using Akai MPK88 via USB and Korg X5 through the MPK's buit-in MIDI interface. The pitch bend works perfectly on the MPK but on the X5 it "jumps around" on random values while moving the pitch bend wheel, and it finally stays somewhere off-tune.

I also had this issue while using a USB to MIDI converter connected to an Yamaha SY77.

This and many more bugs in Apple Mainstage (I've lost hours of work when restarting MS and it closed without asking me to save, just to name another one) makes it difficult for Live use.