1998 Isuzu Amigo thermostat? Or worse?

Minor overheating problems with heater running and sometimes heater blowing cold air for couple of weeks - while driving at highway speed, radiator hose exploded with overheating - replaced hose and it sat for a day - following day, put some coolant in radiator (not enough to fill, though) and drove OK for about 20 miles and began to overheat again - able to stop and let it cool down a couple of times until got it home
thermostat issue? Timing belt? Water pump?
help. Don't want to put too much more $$ into it - just spent almost $3000 on tranny, brake pads, front axle shaft :(