I don't have a tiller on my tractors, but I have used walk behind tillers in the past. They have a tendency to run away from you.
One thing to try is to rotate the tiller tines opposite
to the direction of travel. This puts the tiller tines in opposition to the drive wheels and helps you to move slower.
This method may not
help your over-heating problems and may in fact make it worse if everything is working harder... But is is free to try and nothing is to be lost in doing so. It will be obvious pretty quickly if it does not help.
As far as overheating goes it seems more likely to me that engine heat is building up and heating up everything else including the hydraulic oil. In that case you might want to look at the oil cooler, the cooler fan, the engine tins and block, the flywheel cooling fan, and the hydraulic oil.
1. Is the oil cooler clean and free of debris? Are the fins relatively straight? You can buy a "radiator fin comb" to straighten them. I have seen these for sale at NAPA and on Amazon...
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2. Is the plastic oil cooler fan in front of the pto clutch present and moving? (See #1 below) Are all the fins still intact?
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3. Is the engine (and the area under the engine tins) clean and free of debris, oil, grime, etc. Especially the fins on the heads? Are all the engine tins in place? These tins are absolutely necessary for proper cooling of the engine.
NOTE: There are both horizontal and vertical passages in the heads for cooling air flow. The Vertical passages tend to get gunked up and it's hard to see them with the exhaust and intake installed. De-greasing the engine might be in order.
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4 .There is a fan built into the flywheel underneath the front screen (see #5 below). I have seen this fan become plugged with grass and that greatly reduces the cooling air flow. Trey blowng it clean with compressed air.
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5. Is the hydraulic oil clean and relatively fresh? The Mfg recommendation is to replace it every 500 hours...
Sorry if I come across like Captain Obvious
, but sometimes it's the little things that can cause big issues...