The engine revs

While driving on a straightaway in cruise I will notice a feeling that the engine revs just slightly from time to time. The car does not slow or anything but it just feels like the tranny is going into neutral for just a bit then reengaging. When I take it out of cruise it does just fine that I can tell.

What could cause it to idle at 2000 rpm when first started and stay there?

About 1 month ago, my car started to slip out of 1st gear when starting out. I have to double clutch to get it to stay in 1st (works most of the time). Last night, it started to rev in high rpms as soon as I tap the accelorator. I can not get over 10 mph with out it being between 2500 and 3000 rpm. When I shift into 2nd, it will drop to 1500 rpm then immediately jump to 3000 rpm. I attempted to go up a30-40% incline for approx 40ft and could not make it up the incline. The car feels like it is dragging all the time, and I am unable to get over 55-60mph

I dont trust mechanics in my area. How can I figure out what the problem is for sure before taking it to a mechanic

Engine Mechanical problem
1996 GMC Jimmy 6 cyl Four Wheel Drive Automatic 188*** miles

When my jimmy gets to 4500 rpm the headlights dim and battery voltage bottoms out, and I have no idea what could cause this? Thanks for any help