encoding from flv to avi, audio sync issues

[Ash123]

I seem to be running into problems when encoding from a flv file to an avi file.

I use avisynth to load the file into virtualdub and then encode to an xvid.avi. No probs there.

It's the audio that is the issue. What I normally do is load the original file into GoldWave and then save the wav. Then encode the wav to mp3 using RazorLame. Then mux the new video and audio together in Virtualdub.

What happens is the audio is in sync at the beginning of the new file but slowly goes out of sync throughout the file until at the end it's 9 seconds out of sync. (the audio is behind).

I thought it may be the way GoldWave handles flv files so I used FLVExtract to extract the mp3 but it's exactly the same. The audio slowly falls behind even though it's the correct time length.

I've searched on the net and it appears other people have had this issue but so far no reasonable solution has been found.

Can anybody here help? Has anybody had this experience?

Here's the MediaInfo on the video file. (just to note I've had the same problem with another flv file).

General
Complete name : E:\video1.flv
Format : Flash Video
File size : 259 MiB
Duration : 49mn 20s
Overall bit rate : 735 Kbps
Encoded date : Fri Feb 26 13:23:12 2010
Tagging application : On2 Flix SDK for Dshow_2.5.2.0
canseektoend : No
On2 : Technologies
profile : VP6-E

Video
Format : VP6
Codec ID : 4
Duration : 49mn 20s
Bit rate : 1 596 Kbps
Width : 720 pixels
Height : 405 pixels
Display aspect ratio : 16:9
Frame rate mode : Constant
Frame rate : 25.000 fps
Bit depth : 8 bits
Bits/(Pixel*Frame) : 0.219
Stream size : 563 MiB

Audio
Format : MPEG Audio
Format version : Version 1
Format profile : Layer 3
Mode : Joint stereo
Mode extension : MS Stereo
Codec ID : 2
Codec ID/Hint : MP3
Duration : 49mn 20s
Bit rate mode : Constant
Bit rate : 128 Kbps
Channel(s) : 2 channels
Sampling rate : 44.1 KHz
Compression mode : Lossy
Stream size : 45.2 MiB (17%)