I would start with the basic, run a compression test. Last time mine did this was a burned valve

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Look at your plugs and see if they tell you anything, otherwise post a photo here and someone will give you a opinion. Check the timing with a gun. Check every single one of the vacuum lines, you can't just eyeball. You have to pull everyone one and look for cracks.

For $1000 you can replace the head with a clearwater and a carb from FC products.

98 Montero with cold weather package
96 Toyota Land Cruiser, fully locked Mall Machine :-)