Hi,
Use the -ing for something that is temporary, or new and hopefully will not continue.
Use the regular form for something that is constant, permanent, or not likely to change.
I've been having car problems -- this situation is (we hope) temporary. The problems will be resolved. It's not a situation that describes your car usually.
I'm trying to think of sentence where we use "I am having [a noun] outside of a meal or something to eat or drink. I am having lunch, I'm having a quick bite... but I can't think of a situation where "I'm having a car" would ever work.