A guitar amp at the moment. Happend with just an acoustic. It's the same guitar, (It's an electro acoustic) so most likely my playing. Cables new, seems to be alright. And the stands fine.
So I think it's probaly my playing style (Style? Bad habits more like) or something like a compressor on it?
Well, compressing the signal won't reveal if the problem lies with your playing or the mic.
Can you record at least 2 separate tracks at the same time? What I'd suggest
is recording your acoustic with 1) the 57, and 2) the DI'd signal (straight from
your guitar output to the line input on the interface.
Have a listen. If the signal is dipping on both tracks in the same places (softer
and louder), it's your playing. If the signal on the di'd track sounds consistent,
but inconsistent on the mic'd track, then the problem is either the mic or
the cable.
Oh, and FYI, just because cables are new doesn't always mean they're working
correctly. I got an XLR cable with my SM58 last year, and it stopped working about
two weeks afterwards. So, I resoldered the cable and it hasn't let me down
after a year.
Let us know how you get on!