Sonicator 706 Theraputic Ultrasound

This is a 20 year old instrument that appears to be in very good condition. It has a ceramic barium titanate 1 MHZ soundhead which show no pitting, cracks or other damage. All control keys and displays work and there are no cracks or other signs of damage on the housing. When powered up and the soundhead is applied to the patient, it shuts down after 5 to 10 seconds for what appears to be poor ultrasound transmission. This happens with two different gels. On one occasion I did get continuous US with a combination of medium viscosity get and light layer of water. The head was moved rigorously during this test. I have not be able to get the same results in subsequent attempts. I have tried using the head a various different angles without a notable change in results.. According to the manual the soundhead is to be immersed in water prior to use. There is no mention of gel or other ultrasonic wave conduits in the manual. There is no instruction telling the user to remove the soundhead/wand from water for use. My assumption is that the head can be removed from water for use (some older US machines were meant only for use under water) and can be used with gel. Other users of this model of instrument have reported it worked well with standard ultrasonic water based gels. I am looking for suggestion as to what the cause of this issue is beyond the obvious poor transmission and possible solutions. I am going to continue try lightening the water based gel, but this is unlikely to be the cause of a fairly consistent problem. Were a specific gel type or viscosity required you would think it would be mentioned in the manual. Thanks in advance Bruce Raynor braynor781@aol.com