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Yeah, Xanthro is correct. It's not the adhesive that fails and causes the fabric to sag. The open-cell foam between the fabric and the fiberglass shell deteriorates chemically over time.
When you've had enough with the sagging, take out the headliner, strip it using denatured alcohol to remove the adhesive residue, prime it with KILZ, and then PAINT it with a good quality exterior latex paint. It's a tedious job, but not difficult.
I’m not having problems with my headliner, but I think this is a cool fix, and easy to keep clean!