No mather how hard I tighten them, they still come loose. anything I can do to keep them tight?

Your manifolds should have come with these little tabs that went around the bolts. Once you got them as tight as you needed, the tabs bent up and prevented the bolts from backing out.

Another thing to check is the bolt(s) that keeps coming loose. Is the hole obstructed? If there is something in there, they are not going to get as tight as you need them.

Just a few ideas off the top of my head. Oh yeah, the tab things can be picked up at the dealer and could possibly be picked up at your local parts place.

What about that Thread lock glue stuff. Anyone tried that?

Too much heat will break down the threadlocker.

Try using new lock washers if it is stock manifolds. They do "wear out" over time