My 2000 Lincoln LS 3.9 V8 is turning on and then it pops real loud a few

[Hirlanda]

times and turns off. HELP? I had a few mechanics look at it and one said it was definetly the spark plugs so i got them changed.. then it still has the same problem and then another said it was the air intake control valve. Got that changed and it didnt solve the problem... Another mechanic said it is the timing belt and that it will cost me $2400.00 to fix. I dont know what to do..... if i pay that and it still has a problem then I wont be abvle to afford to do anything else.....Does anybody know about these Lincoln LS's all the mechanics that ive called arent familiar with them... i can keep the engine running if i keep my foot on the gas but it makes awful explosion noises in the motor somewhere... HELP