vitz weird problem??

[rider2012]

hello!last year brought vitz 08.i dont know what seems to be problem.when i drive a rough road the car bumping and when i drive in slim(soft)road the bumping disappear..first i thought suspension then i realize the tyres beacuse tyre air pressure usually 30..in 30 air pressure the car bumped.and when i add air pressure 26 to 28 the car bumping disappear..needed help.....????what should i do?go for tyre our suspension... waiting for ur replys..our go for low profile tyres.my car tire is hard tire