Frenchman,
-The aforementioned probe might help
-Fans do an amazing job of cooling your system... run a small fan or two in your hood to circulate air
-Consider raising your hood an inch or two to allow for more air between your light and the tank top
-If you're not using a sump now, consider adding one... more water volume and you can run a fan over the sump to cool the water. This can be a big help, if you're using a smaller pump as a return (i.e. a big Rio 3100 would probably generate almost as much heat as a 250w MH)
-You might want to consider cutting back the amount of time your lights are on each day and/or bringing them out-of-phase (i.e. lighhts off in the heat of the day and lights on at night).
Remember that some critters are Ok in higher heat, provided there aren't big shifts in temp. When I lived in a hotter clime, I fought a battle to keep my tank @ 78 degrees for a long time, but finally decided to switch to ~81 degrees constant. I found that this was better in the long run.