Truck runs cold after warming up even with thermostat replacement please help
 

So I inherited a 98 Silverado from my dad with minor problems but one of them was it runs too cold like 150 degrees so I thought I should replace the thermostat and I did now it warms up to 200 degrees but right after falls down to 150-170 and stays there

You might test that new thermostat for accuracy. Based on your story it sounds like the stat is opening properly but then opening too much and sticking. "New" doesn't always mean good - could be defective... OR you need a warmer stat..