One thing that could cause this is a bad charging system/weak battery.

If the battery is being drained, the voltage could drop low enough until the PTO kicks out.
Without that draw, the battery may regain a bit of "charge" until the voltage rises enough and the PTO kicks back in. Repeat....

You might check the resistance of the PTO. it should be in the neighborhood of 2.5-4.0 OHMs.

Also, if the air gap has gotten too wide, it could possibly exhibit that problem. Some are adjustable and some aren't.

I don't know what PTO you have, but this manual can give you some hints-

http://www.warnernet.com/pdf/819-0458_P-1097-6.pdf

Wait, Wait !!!... Methinks I detect the pounding of hoofs on the turf in the distance.. Galoop, Galoop!
Yes - 'tis that of the masked Antlerhead ready to hold sway over all shiny green problems (and then some)...

Make Way, Make way, make way....lest ye too be trodden upon..