pr3dict

I have a 1999 Hot spring sovereign and The other day I was in the tub and realized the hot water stopped coming out of the vent at the bottom of the tub. I didnt think anything of it and left it alone for a day. The next day the LED that shows when it is in range of the thermostat was off so I turned down the thermo and realized it had lost hear. I reset the breaker and now almost a week later the tub is still losing heat. I put a hose down the main stack because the water was low the other day and could have possibly sucked up some air. When I put the hose in the bottom vent just makes little bubbles faster. I don't believe it's air locked because the pumps still work. HOWEVER The heater keeps tripping it's little red button on the back. I pushed it in and I feel the metal pipe and it seems to get warmer. I walk away and come back about an hour later and its not heating again. Does this mean there is air in the heater itself? Or something else maybe?

Hi my neighbor is moving and offered us a free 20 year old hot springs sovereign. Free always sounds good but am I destined for failure with a unit this old?