Check the belts?

Check belts like they said but also see if there are grease fitting on the shafts where the blades are connected. Some Sear Mowers have them and some don;t. I have seen the bearing on one freeze when it gets hot and stop all the blades from turning.

I don't know much about the specs of your mower but I will tell you what happened to mine. I have a Wolf electric mower that has some sort of a auto brake mechanism once the power is cut - to stop the blades spinning after power shut-off. That auto brake somehow did not work properly from day one and everytime I used the mower it would run fine untill the friction causes the shaft to heat up and gradually the brake would stop the rotation of the blades making the thing come to a stop. Once cooled down a bit, it would run fine untill process repeats itself. Long story short - I removed the motor from the housing and had my good friend press the bearing type brake/clutch from the shaft and re-installed the motor. Chucked the brake/clutch in the bin and never had a days problem again. The brake looked like some sort of a bearing (disk without the actual balls) with some sort of brake lining material on the outside of it. It was supposed to lift as the motor picks up speed thus making the motor run free. As soon as you release the power the disk would fall into place and stop the motor spinning the blades. Maybe this helps.

You did not mention just what you are mowing with your riding mower. Typically mowing tall grass (where grass accumulates under the deck) will cause the "symptoms" you descibe...that and/or the bottom of the deck has accumulated buildup from not being cleaned.

But, let's assume that's not the problem. Again, you did not say...but my guess would be that the riding mower is equipped (rather than blade engage lever) with electrical (finger-switched) mower drive engagement. If so I would recommend checking the clutch gap...which has probably grown too large. Set the gap back to factory spec and your problem should be eliminated. Belt wear/stretch and spindle lube can affect mower performance but will not typically produce the symptoms (the failure mode) you described. But, you could check the belt tension adjuster for correct gap...and make sure any tension adjustment spring is installed and tensioned according to owner's manual.

Finally, check the interlock switches (seat switch and PTO/mower-activate switches) for correct alignment, cleanliness, tight connections. It is possible for intermittent breaking or closing of these switches (due to vibration, heat, or arc shorting) to produce the on-again, off-again mower functionality as you described.

If these do not fix the problem, you'll probably need to trace to a faulty (intermittent) relay in the mower clutch activation circuit. Another thing, how well is charge on the battery holding up? If battery recharge is working nominally, we can probably eliminate too-low dynamo output as the problem.

Hope this helps. Let us know. In the meantime, my money's on faulty clutch gap or function.

You don't say how old this mower is, but if under a year, it should still fall in the original warranty (and if you sprung for an extended warranty, that should cover another year or two). Sears has a good reputation for standing behind their warranties, so I'd let them have a crack at fixing the problem first. They won't fix stuff you broke trying to fix it...