Problems using IntPtr and Interop services to get image from unmanaged code

[cewood]

Hi,
I was hoping someone could help me with the following: I'm writing
code to try and get a image from a USB camera mfg. dll.

Here's the dll function:
SDK_API MTUSB_GetCurrentFrame( DEV_HANDLE DevHandle, int FrameType,
unsigned char *Buffer );

Here's my C# wrapper:
public IntPtr GetCurrentFrame(FRAME_TYPE FrameType)
{
int FrameTyp = (int)FrameType;

if (MtxGetCurrentFrame(_devHandle, FrameTyp, _pImage) < 0)
{
MessageBox.Show("Error trying to get current frame
buffer. Frame grabber running?", _camError, MessageBoxButtons.OK,
MessageBoxIcon.Error);
}
return _pImage;
}

which calls into
[DllImport("c:/Windows/system32/MT_USBCamera_SDK.dll",
CallingConvention = CallingConvention.Cdecl, EntryPoint =
"MTUSB_GetCurrentFrame")]
//private static extern int MtxGetCurrentFrame(uint DevHandle,
int FrameType, ref byte Buffer);
private static extern int MtxGetCurrentFrame(uint DevHandle,
int FrameType, IntPtr Buff);

Before calling I allocate memory for the IntPtr like this:
_pImage = Marshal.AllocHGlobal(pixelDepthBits * _maxX * _maxY);

After the dialog box closes I free the memory on the heap like this:
Marshal.FreeHGlobal(_pImage);

It's weird because if I do individual clicks to "snap" an image
everything works fine. An image is grabbed from the USB camera and
displayed in a PictureBox.
But if I do the same thing using a timer, the application will run
from 2 to 15 seconds before throwing the following error:
An unhandled exception of type 'System.AccessViolationException'
occurred in USBCam.exe

Additional information: Attempted to read or write protected memory.
This is often an indication that other memory is corrupt.

I'm using Visual Studio 2005, with the .NET 2.0 framework on Windows
XP SP2. Any idea on what I may be doing wrong. BTW, I get the same
error if I do an unsafe call using a byte array and the fixed keyword
to a byte pointer.

Please advise.
-Chucker