Can anyone throw some light please.

I am converting avi (Premier/Avisynth/Tmpeg12a) to Mpeg1 using the 'unlock' template. I reduce the resolution of the exported file from Tmpeg and export. Fine so far.

On playing back the new, reduced size mpeg1 file using mediaplayer (6.4)the player starts to contact activex/microsoft looking for a codec. It is unsuccessful and reports back as unable to find appropriate codec.

However, on first playing it plays at the correct resolution. All subsequent attempts to play the file result in it playing back at 720x540!I just want to be able to produce small resolution mpeg files.
I have mailed an example file to Sefy (who is also using mp6.4)and he is able to play fine at the low resolution and so I am 'missing' a codec.

Can anyone please advise what might be happening here?

Cheers
Brent