Robert - our oil one doesn't require electric. The oil is gravity fed into a regulator (silvery oval thing on side of aga) and then wells up into a dish with three rings inside the aga.
In terms of oven temperature, one thing we used to find (forgive me if I'm teaching you to suck eggs), is that we had to cook slightly differently with the aga compared to normal hob/oven. If you do most of your cooking on the hotplates, then the heat stored in the aga will quickly diminish, and you will find the oven temperature dropping. It is recommended that 80% of your cooking is inside the ovens, and only 20% on the hobs. So for example, we get our spuds boiling on the boiling plate, then transfer to the floor of the roasting oven. Similarly, cooked breakfasts are started on boiling plate, then frying pan sits on bottom of roasting oven. Keeps the smell out of the kitchen, and makes the juiciest breakfast you've ever had!
Also, as you cook the heat stored will diminish, so for example after a few pots of coffee with the lid up, then soup mid morning and a big sunday roast, ours would not be as warm as in the morning.
It might be that one of your wicks is going out, but this shouldn't happen if it's just been serviced.
Hope that helps,
Neil