Is it usual for intake manifold gasket replacement to last only 26,787 miles.

There is no usual or unusual for the intake and or gaskets on this 3.8 L engine. I think that is the eng. that is in this car, 4 years since the repair, that would be around 2008 so that means you only got 1 MORE year out of the FACTORY gaskets! Gaskets have been improved since that replacement was done. Have intake manifold checked and new gaskets installed IF that is where the leak is! Also dont put DexCool back in it, use the good old green stuff. That is better anyway i have found! Have the installer check bolt torque after a couple of weeks of driving. Now, water in oil /external leak? Or vacuum leak.

Age has more to do with this rather than mileage

use factory repl gaskets as they are better than the aftermarkets