I have an 89 Firebird 305 5 speed. The car runs fine when first started then after 15-20min or so it stalls. The tach kinda jumps around a bit and sounds like it's missing then it stalls. The car starts right back up, but stalls again if you don't give it gas. I've replaced the the distributor and the spark plugs look ok. The car was throwing a code for a bad MAT which I also replaced, but it did not help. Any suggestions to what the problem might be would be appreciated.