How do I fix my Stoves 600 SI DO DLA?

I have a stoves 600 SI DO DLA gas cooker that is taking too long to cook meat in the main oven, i have bought a temperature gauge to check the temperature and it does build up close to the correct temperature in relation to the gas mark.
Is is possbile that the temperature is not stable, as an example a small joint of beef that the local butcher told me would take 45 mins at 180 degrees (also checked this against a Jamie Oliver recipe for cooking beef) took 1.5 hours.

Paul
November 2011