Open the bottom panel and clean out the dust about once a year.

If you use it on a desk, prop up the rear by 1/4" or more to give it more airflow under the bottom panel.

Most laptops are actually designed to run in an air conditioned space.  Don't use it in some 90 degree Fahrenheit room.

Set it to low power mode. Here's how you do it in from the command line in Terminal.app

sudo pmset -a lowpowermode 1

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Which MacBook model is this about? Do you use it with the lid closed? 

I've worked with multiple generations of MacBook pro - they all had (dual) fans that were keeping the device cool under high load. Kids used to play games on them as well, with the fans on constantly at high speed. Nothing ever happened like you're mentioning.

<edit>
Apple says it can be used in a room with 10-35 C (50F - 95F), keeping it on a flat surface, not covering ventilation openings etc.

https://support.apple.com/en-us/102336 

Thanks,

The Macbook is pretty thin. 15 " screen. I keep the lid open…

It heats most when I plug in an external firewire usb hub, which I need for a mouse.

It does not have a USB  port, just firewire.

I don't use a mouse now, but it would be nice for games. Games heat it lots, and the screen pixels splatter sometimes. I power it down when I go away to do something. It has pitiful ventilation.

>> It heats most when I plug in an external firewire usb hub

That's not good, I would see if you can get it replaced?

A mouse is a low power device and just using it with a hub should not drain power that makes the fans spin up.

Or is it a powered hub and you other devices connected to it? It should be a Thunderbolt hub btw now a Firewire hub.

Do you think that once it's gone over a threshold of heat damage, it is irreversible?

So far, this morning, with the bottom raised, there has been no pixel shimmering. Just  heating over the keyboard. Better status than yesterday, for sure.

Instead of asking Bilzi Mac for another screen replacement, I might ask for a MB  replacement and HD transplant? - 500 GB. By Monday, I'll know for sure what to do. I've got another hub for usb mouse. i'll try that out.

Thanks

I doubt that the monitor is damaged at all.  The “pixel shimmering” is generally caused by faulty data in overheated chips, causing data signal corruption. If the chip heats beyond that threshold, the graphics should completely freeze up.  

If it didn't heat enough to melt the graphics chip or the packaging, then you only shortened the lifespan of the GPU by a little bit.  It should still mostly last well beyond your typical owner usage lifespan.

Have you opened the system and cleaned out the dust? 

I don't have the equipment to open it. I might take it to Bilzi Mac tomorrow. Good Mac folks.

You just need the pentalobe screwdriver.

I might have some time available tomorrow. get back 2 u