Inaccurate casting results

After weeks and weeks of casting, I feel that my results are inaccurate. This is due to me using an older manual Kiln that isn't able to 'hold' at specific temperatures for the required period of time.
Initially I felt that the old kiln would suffice as the workshop does its fair share of lost wax casting and it worked perfectly, however I now feel the need to purchase a new as resin casting needs requires a more accurate burnout.