Running hot

So my car was able to hold conventional 5-30 for about six months around 176,000 miles approaching eight thousand miles.
It has now a 500 to 400 rpm idle.
Instead of a 700 to 1000 idle.
I have used a 10-30 high mileage on my car it runs better but it still gets hot.
I check my fluids they are okay.
However, the car now slowly creeps past half.
I try to treat the car good and don't allow it to overheat.
Another thing If useful;
I recently changed my alternator out and my lights stopped dimming, it had held heat to normal temperature according to my gauge.
But now they are dimming and and to my best belief it is having power surges where everything becomes bright but then dims again.
Where do I start?
Is this a wiring Issue?