We went through this with our now 6 year old male, Auric. It was so intermittent that we had trouble getting a good grasp on the problem. Urinalysis finally revealed that, although he did not have a UTI, he does have a chronic condition (I can't remember the name!) where his urine crystallizes too easily, causing discomfort when he would urinate. No blood, no issues with quantity, no odor. The solution has been to switch him to a prescription food (Hills Prescription Diet C/D). Yeah, it's a little expensive, but not cleaning cat pee up several times a week has been priceless.

Going to the vet is a good idea, if only to rule out medical causes so that you can focus on behavioral ones.