Ignition or induction. Start with the simple stuff; fuel filters, fouled spark plugs, coil packs (I don't know whether your car has a distributor or coil-over-plugs), crankshaft timing sensor, clogged air filter. dirty fuel injector nozzles, old gas. Then move to more complicated stuff if that doesn't fix it; clogged cats, sediment in fuel tank, timing chain problems. If it isn't making destructive noises, it's probably something relatively simple. If it's getting good spark and good fuel flow, then it's something more complicated. I'm sure you've checked the obvious stuff like a half-tank of diesel by mistake.
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