Toro LX420

I have an '06 model Toro LX420 lawn tractor with 120ish hours on the meter. First six years were trouble free with only maintenance needing performed. After that things went downhill quickly, broke the deck belt and hasn't been right since. The deck well run normally until disengaging the belt, then without fail it will throw the belt with no pattern to which pulley it comes off of. Checked runout and found excessive on one of the spindles, replaced to no avail. Have had the deck off and on countless times, see no evidence of bent deck or misaligned pulleys. Dropped one size on belt and worked for two mowings then broke. I own an automotive repair shop so am no stranger to mechanical work, but this thing has me at wits end. Any help or ideas would be greatly appreciated.