do head gaskets just blow like that?

madalas, after an overheat (usually very bad) incident, dun nasisira ang head gasket. minsan, but very unlikely, na nachachambahan lang.

eh in your case, the overheating actually happened after your engine bogged down.

... eh nag flush ka ng radiator (bakit kaya nila ginawa to?)... kaya medyo confusing na. ang hirap i-isolate kung yun ba talaga ang cause ng overheating.

ganito na lang muna.

1. bigla bang naging puti ang kulay ng exhaust gasses mo?
2. may langis ba sa coolant?

kung oo, malamang you just suffered a blown head gasket.

otherwise, like what others have said, you might want to start with the simplest (and usually least expensive) first. elimination process, until you find what's causing the problem.

mahal din at matrabaho magpalit ng head gasket ha...

good luck!

p.s. look for another mechanic please.