2001 Chevy Malibu engine skipping

Engine problem
2001 Chevy Malibu 6 cyl Two Wheel Drive Automatic 139k miles

2001 Malibu. 139,000
problem 1.I am getting that brown foamy, gooey stuff in the oil. Since I'm an old guy I think it is the head gasket leaking coolant into oil. There is no black, white or gray exhaust.
Q. #1 can anything else cause that?
Q #2 if I change the oil every 1500 how long before the engine goes? In the old days I've driven cars 2 years with that goo. But that was steel blocks.

problem 2, 3, 4.
Also, same car. When cold the engine makes a high pitched whurring noise that goes away as it warms. At lights it idles fine in drive but has that whurring noise in neutral. Seems to have great power on take off and will hit passing gears quick. However, sometimes as I'm driving at 55+, the engine just starts skipping suddenly, the car jerks and "check engine" light goes on/off on/off. Need to pull over and idle it 2 minutes then all is fine again. On steep hills the head lights and dash lights get dim and blink then the car has no power and seems to skip. I suspect the belt is bad and slipping. May that have been the original cause of all the above? I should have had it changed months ago?