mac freezes, then question mark at startup. HD problem

last months mac is getting much slower after a while (just 1 app open) usually photoshop or final cut. now and then it would freeze, screen blacks out, nothing i can do but push the startbutton.

that's when it starts up with a "?" folder blinking.

Boot from install disk works, i can see my external drives, but usually no internal HD to be found.

Then the next morning, mac would start up & everything is fine.

until next freeze..

i' ve gotten about 7 in the last 3 months.

last time, before today, my internal HD did show up when booting from the install disk. so I ran repair disk from disk utilities. checked everything and got a green line at the bottom saying: The volume Macintosh HD appears to be OK.

I then choose the mac HD as startup disk, restarted & all was fine.

Last night, it was getting slower again, but i had no freeze and i shut down normally.

This morning, 'question mark' flashing on my cold mac!!

(i was thinking it was getting to warm and therefor frooze)

no internal HD in disk utilities, i do see my ext HD's

are those symptons of a dying (dead) Hard disk ?

Or could it just be the cable connecting, since disk utility said all was fine with the HD.

or is it too old almost 4yrs and do i need to get a new one?

ps. i do have a back-up. (time machine)

please help.

sincerely, Cy.