A/V Sync Issues after Conversion with StaxRip

[fpx]

Hi,
I am converting some old VHS PAL types. I capture the video using an Elgato Video Capture USB dongle. This results in an MP4 file (AVC video, AAC audio). I then transcode the file using StaxRip to reduce the file size and to crop the corners, again into an MP4 container. Most of the time, this works fine, but occasionally I loose A/V sync by a couple hundred milliseconds. I.e., the original MP4 as produced by the Elgato software plays fine, but the StaxRip-converted file does not (using VLC in both cases).
Looking at the MediaInfo output, I see "Frame rate: 25 fps". However, while it usually says "Frame rate mode: Constant", it occasionally claims "Frame rate mode: Variable" with odd minimum and maximum frame rates such as a 3.289 fps minimum or a 30000 fps maximum.
When it says "Constant", the conversion is always flawless. When it says "Variable", the conversion is sometimes fine (or maybe the A/V discrepancy is just neglegible), sometimes not, and I get a delay of up into a seconds range (IIRC the video is then always ahead of the audio).
I am attaching a StaxRip log file from a "bad" conversion.
Do you have any suggestions what I can do differently to fix these occasional glitches, or at least to further diagnose this issue?
Thank you,
Frank

[davexnet]

I've had good luck in the past using fpsnum and fpsden with the FFvideosource. For example:

FFVideoSource("Z:\Movies\Hitchhiker-2.mp4", cachefile = "Z:\Movies\Hitchhiker-2 temp files\Hitchhiker-2.ffindex", fpsnum=25,fpsden=1)

[fpx]

Dave,
indeed, that seems to work. Thank you very much for the suggestion!
It just seems a bit inconvenient, or maybe I'm doing it wrong. After loading the original file into StaxRip (which demuxes the file and creates the index file), I then create a new .avs file with the above content and re-run StaxRip using the .avs file as input. One effect is that StaxRip's crop feature does not work well: it just shows the first frame but refuses to seek. So I have to copy the crop values from the first run.
Is there maybe a way to set 25 fps as the default? I've tried adding a call to the "AssumeFPS" function as "code to add to the top of scripts", but whatever I do it complains that the parameters are invalid.
What would be the proper syntax to use, or is there another means to do that?
Thank you,
Frank

[davexnet]

I'm not that well acquainted with Staxrip, so I'm not sure if there is a way to add these defaults.
Perhaps somebody with more familiarity will respond.

[sneaker]

I've tried adding a call to the "AssumeFPS" function as "code to add to the top of scripts", but whatever I do it complains that the parameters are invalid.

AssumeFPS() is not the same as using fpsnum/fpsden in ffvideosource so that's futile anyways.