i think my injectors are blocked on my 2001 astra sxi?

my injecters seem like they are blocked as in high gears on low revs my astra sxi is hesitant and juddering. could this be a blockage or a faulty air flow. i've heard somewhere before that the mark 4 astra's have a problem with the oil going hard on cooling causing blokages too.. does anyone have any idea's how to solve this problem. also what materials would i need to solve.. ta..