Mower deck 725 Front mount 61" Power lift won't maintain cutting height

Title pretty much describes my problem. It is a 2004 year model that I bought new in 2006. The last couple of years after setting cutting height at 2 1/2", when I mow for about 30 minutes on a little bit rough yard, the cutting height slips down about 1/4" and sometimes the electrical switch on the power lift mechanism will shut off the electric clutch. I then bump the height adjustment back up to the original height and everything is fine for another 30 minutes. I realize this is not an earth shaking problem, compared to everything else, just a nuisance. Only 360 hours on the machine. Everything seems to still be tight. Any ideas??