99 Buick Lesabre won't stay running

I have a 1999 buick lesabre, just purchased, kept having a problem staying on unless i consistently put gas to the pedal while driving otherwise keep it moving, if sitting idle it shut off, oil was black so took to jiffy lube got total oil work up costing almost $200. check engine light was on they checked it n said mass air flow and spark plugs needed replacing, got them changed but the person who changed spark plug wires had firing range wrong so car was shaky, he did something to it and car drove smoothly no shaking no nothing,drove round the block couple times..nothing so proceeded to go to store an about 10 minutes later i noticed the car was quiet and slowing down, realized it had shut off..i tried giving it gas while turning on like ive done before that got it running and nothing. so called guy bk out he did something and then the car kept making screeching type sound he claimed starter problem. got starter tested starter good. now another guy checks and he says could be flywheel but couldn't say have to take to shop which i have no money for. finally another guy takes a look and says my harmonic balancer isn't moving and it should be turning even if the car won't start and it might have locked the engine; however i read that the engine knocks prior to locking up which it never did ever. So in your opinion what do you think since you seem to know your stuff. thanks. as a woman this is driving me nuts.