"fishtailing" is left-to-right, and is an indication of either a poor release, or incorrect spine... cheapest thing to do would be to buy a cheap tab and see if it changes at all, better or worse.

if no change, then spine is the culprit.

"porpoising" is up and down, and is an indication of poor bow tune, incorrect tiller, nockset out of place, etc...

tell us what way the arrow kicks when it leaves the bow, and we'll be more help specifically.