89 surf idle speed increasing

[callumw18]

Have a 89 hilux surf 2.8 diesel non turbo. The idle speed was running a wee bit low ( wasn't to bad once warmed up) so i tensioned up the cable so it now sits at 750ish rpm. But when the car gets warm after 5 min of driving the idle gets stuck at 1200 rpm. I have been into the engine whilst running and you can pull the back and cable through pretty easy. Lubed everything up as i thought could be sticking but still has the same issue.
What could be the problem here?