Thanks,Stephen!
I Cannot confirm that is just for hi-md discs.i tried to record to formated 80min.md disc and i got the same problem.
There's one thing i did't mentioned in my previous post - i went to the local repair service(not sony) and asked for help.they fixed it!they said that all they needed is just to clean the lense.but after a couple of weeks,i got the same problem,and when i returned to repair guys,they did't accept my md anymore,cause they're not sony and they had spend alot of time fixing my md )
Actualy i don't think it's the cleaning what helped.it couldn't get dusty in 2 weeks again.hmmm...