I have an 88 camry without A/C and automatic temp system. At times it won't heat. I've checked the hoses, stat, heater control valve, etc. When it quits blowing warm air i can hear something in the heater box moving, then it blows cold. Is someting opening the air vent into the system? I pulled the control panel and sprayed the switches with WD-40, that helped for awile. There is 2 little control boxes, 1 behind the radio, and 1 under the heater box, all electronic. I didn't see any vacuum hoses to any of the heating system. Anyone have any ideas? Thanks