Can someone tell me what is wrong with my 1996 ford probe gt?

My ford probe runs. When it is just sitting there running its hesitates a little. We replace the spark plugs, muffler, and fuel filter we can't figure what is wrong can anyone tell please|||find out the condition of your engine by running a compression or leak-down test...|||pull the codes from the computer, if codes are there follow the code diag. If there are no codes, check and clean the MAF sensor and also check for a vaccum leak from all vaccum sources on the engine.