I have a trane xr80 about 10 yrs old.It is a downflow gas furnace with a/c.The problem I am having is the furnace will fire right up the 1st time and work for any given amount of time, then it will in the middle of working fine stop heating and start the startup process all over again. It will go through all the steps fine(blowers on,igniter is glowing) but when it clicks for the gas to come on nothing... then it retries and will just keep retrying and won't ever shut down unless I kill the power or adjust the thermostat. I have cleaned the flame sensor( it was loose in the porcelian collar that was around it) I tested the voltage to the gas valve solenoid had 24v when it was trying to start but wouldn't. I changed the thermostat. On a side note we just bought this house and don't no how often the filter /chimney was changed/cleaned. Anyhelp would be appreciated.

ALso no error codes. Its blinks slow/steady when not in use and a faster/steady blink when a call for heat,even when the furnace starts acting up there are no codes.