Sounds like your vet started with the right steps of physical examination, blood and urine testing. With the history of recurrent, episodic pain leads me to several concerns. Some of the differentials that would be high might include: neurologic disease, immune mediated disease, infectious disease, high blood pressure and cancer.
Neurologic disease might include slipped intervertebral disc disease, inflammatory or infectious diseases, vertebral instabilities or tumors. Unfortunately, MRI and CSF analysis are going to be the means of identification. These, while very safe, are expensive.
Immune mediated diseases are autoimmune inflammatory diseases which can occur for various reasons. Inflammation of the nervous tissue or muscle tissue can lead to the signs you are describing.
Infectious causes may include things like a kidney infection. While urinalysis is a great way to start, sometimes culture of the urine may be indicated. Ask your vet, some urinalysis test results may make culture either beneficial or a waste of time. Infectious meningitis, myositis, or joint infections can cause recurrent episodic pain. So might tick borne or ricketssial diseases, as hazelpack run suggested with Lyme disease. Liver infections or even heart infections may cause this. You may want to ask your veterinarian about a therapeutic antibiotic trial.
High blood pressure can be very uncomfortable. This is a simple test - if your vet has the equipment. The machines for reading blood pressure are fairly expensive.
And unfortunately, for situations like this cancer does need to be considered. For this reason, among many, further diagnostics and investigation is an excellent idea. Remember that many 'cancers' can be affectively treated.
Best of luck, I hope this helps.
