Need your help! - Audio out of sync when mixing

[solo.guitar]

I have 2 tracks.

Both are 16-bit/44.1

The first is a MIDI track that was run through a VSTi and rendered as a 16-bit/44.1 stereo WAV file.

The second is something I recorded over the first, and it is also 16-bit/44.1, but it is mono. (I even tried recording it in stereo and I still had a problem).

When I play back the audio, they are out of sync. They start off okay, but after several seconds they become out of sync. The second track plays faster than the first.

I am using Adobe Audition 1.5's multi track. I even tried it in Reaper, and I still have problems.

I have ASIO4All installed. I've had it for awhile but never had this problem.

I even tried upsampling then downsampling again just to see if that would work, but it didn't.

What gives?