I have a Marshall MG 100HDFX amp and I want to record thrash metal.

I have a SM57 microphone.

I connect the microphone to my Line 6 Toneport UX1 into my computer.

Whenever I start to play, and strum fast, it sounds like crap and the strums sound so seperate from one another like its not enough gain or something, but it sounds fine in reality.

Tro to experiment with moving the mic around, According to the IR i use, the mic is pointing straight into one of the speaker with only 1 inch (3cm) between mic and cab.
I believe that means you have a low-quality connection from your mic to whatever it is your using...Although i am probably wrong.
John Petrucci

The one and only god.
I find the best sound for micing electric guitar comes from micing it in between the dust cap and speaker cone that gives you an even balance between the lows mids and highs. Or if you have a good sounding room you could try distance micing about 3 to 5 feet just remember the sound of the room will get in the recording as well.