Speed death with too much current.

Hello, I have a little problem with our 323F of 99. And I have had the problem for about a week, that when I turn on stronger consumers, such as ABlight, F-light, rear heating, ventilation level 4, then my speed starts to shake properly. that jumps from its usual 900 u/pm up to — almost off– 400 u/pm the engine and the whole car then starts to wobble so strongly that you can hardly sit still. Very funny. The speed remains quite calm as long as I have none of the strong verbs on. What else can it be, that the speed spins when I have other consumers. Mazda wants to keep the car there and sift through everything until they have found the problem. What would that cost they couldn’t say exactly before. Human I’m not a money-shit. Can you tell me what it could be or what it would cost them to do? could I check myself?? The engine runs completely normal when the consumers are out and when they are on I can also give normal gas and then it drives quite normally without jerking or something. Can you help me??