There are several diseases which can cause loss of muscle strength. By PMR if you mean polymyalgia rheumatica then this would not be expected to cause loss of strength as what you describe.
One possibility could be Myasthenia Gravis. This is an autoimmune disease and would cause weakness. It would tend to cause early fatigue and rapid weakening which might improve sonme with rest.It would affect the biggest muscles to a more noticeable extent such as the leg muscles.It is caused by antibodies to the acetyl choline receptors which receive neurotransmittor signals at the neuromuscular receptor sites. This can be tested for by testing for the ket antibodies that cause this disease.
Muscle weakness can be caused by neuropathies. Often neuropathies have both loss of strength and sensation. Some neuropathies are more predominantly affecting strength however. ALS would be a potential concern of a motor neuron neuropathy. (ALS =Amyotrophic Lateral Sclerosis)
Muscle weakness may be caused by primary muscle disease such as muscular dystrophy. Polymyositis is another disease caused by inflammation and breakdown of muscle tissue.
The loss of strength might also be caused by a central nervous system problem. I would think this less likely to lead to a 15 minute period of weakness but this category may need to be considered as well.
I would recommend that you consult a neurologist for further evaluation. I think that this would be your best next step for further testing in order to obtain further testing.
I hope this information is helpful for you. If you have further questions then let me know and I will be happy to get back with you. If my answer has been helpful and to your satisfaction then please remember to press the "ACCEPT" button. Thank You and Best Regards,
Anthony Bray MD