Hi,
Frustrating, I know.
But, it sounds like the engine protection system is working as it should. It senses overheat and de-powers the engine to protect it then re-sets when the ignition is cycled off.
When at idle and low speed, you burn less fuel so there is less heat generated. When you power up you "make a bigger fire", so to speak, and the heat quotient goes up dramatically.
But....what's wrong?
Since you say the pee stream is ok it looks like the pump is good. Right?...
....BUT...I've seen pumps with only one impeller vane still intact make a decent looking pee stream so I wouldn't discount the pump as being "weak" and not supplying enough water to keep that bigger fire under control.
If it were me, and considering where you are....
....I would pull the thermostat and have a look in there. I would also test the stat in a pot of water on a stove to see what temperature it opens at. I actually test NEW tstats like that because I have found many to be bad right out of the box.
If there is a fairly high degree of mineral/salt/corrosion build up inside the housing, then you'll probably want to do some cleaning in there.
And, more than likely, you would be well served to pull the water jacket cover and clean in there as well. Items 8 and 9 in the link below.
I recommend replacing the zinc anode and attachment screw
(items 1 and 12) as preventive maintenance if you go in there.
That screw can sieze and snap off in there so remove with care.
If funds allow, I would actually replace the tstat and gasket as well as the water pump even if all looks ok. In your situation it's always good to keep the used stuff as spares if they seem serviceable. The gaskets and/or seals don't cost much so buying extras to support that strategy would be a good idea too. Your choice.
If you go with replacing the pump instead of just inspecting it, I recommend a COMPLETE NEW KIT (item 2 in the link below) as opposed to just a new impeller and gaskets (item 1).
While a complete kit costs more, it ensures that the new impeller, wiper cup and housing all fit and function as one and will give you 100% efficiency of moving water for cooling.
A new impeller installed into a used wiper cup will rapidly wear down to conform to the old cup. This decreases pumping efficiency.
Also, always use the new drive key with the new impeller because, even if the old key looks ok, it IS definitely worn to some degree. Very close examination of it will bear that out.
It does seem to me that the engine is likely overheating so you should avoid using it until you get it sorted out. Because, even if the protection system is working well, you can still do small amounts of damage each time she gets too hot.
I hope you get it fixed up without too much trouble.
Good luck.
