I was refused entry to the US several years ago as they became under the impression that I was trying to work illegally( which was not the case). Since then my passport has been flagged and every time I have made and attempt to cross the border- I have been stopped and drilled with questions, even though I have successfully travleed through 3 times now. How long will my passport be flagged for?

From another thread posted by you... now I assume you're trying to enter US with Canadian passport.

If you have been in US with visa waiver program, and you overstayed, this might be one of the reasons. If you did not overstayed but entered US again in only a few months after the first trip, it definitely caused suspicious that you must be working illegally in US.

You need to have proofs that you have secured job in Canada, and the visit to US is vacation or visit family. If you cannot prove ties to your homeland, immigration officers may not let you in.