Glow Plugs?

Hi everyone, i own a 1990 patrol TD42 cab chassis and for 15 years i owned it its never missed a beat , the light and click used to go out in sync and start on first crank ...recenlty though the light goes out after 3-5 seconds and the click around 20 , in the morning its starts first crank after the click and same in the afternoon leaving work but when you drive a bit then stop for fuel or at the shops, it doesnt want to start...it cranks over until the battery is almost gone before it fires up again...

Its done around 445 thou km now and i had the pump and injectors done over the years but never the plugs , could this be the problem or would it be hard to start even in the morning and arvo if it was , i cant seem to work it out.

any help would be appreciated