It's possible, but unlikley to be something else unless you had recently changed the rad or something similar causing the airlock. If its the same before and after coolant change then doesnt sound like its an issue with the cooling system itself . Check for any leaks on the rad, they sometimes get cacks and only leak when it gets hot, just to make sure.
After that then your onto rebuilds. Clinic can give you an idea of that as you say, but i'd look at replacing the rods and pistons while its apart. Stock pistons are pretty crap and known to suffer ringland failure, stock rods will be fine if your staying near stock power, but anything aproaching 400 and your putting them at risk. Then other options like closed decking depending on how far you wanna go and what your budget is.