Starting problem / running rich

My 2002 Astro van (150K mileage) was difficult to start when hot - had to turn over multiple times before firing up (no problem when cold) and would occasionally misfire when idling. Then the 'check engine light' came on. Work done to date includes - replacing catalytic converter (which was blocked), replacing shaft in distributor (which mechanic said was wore and wobbling), new plugs, wires and rotor, complete injector cleaning (at Chev. Dealer). The starting problem has not been resolved and the 'check engine light' came back on. Diagnostic check revealed codes P0172, P0175 - explanation given as System too rich - Banks 1 and 2 respectively and code P0300 - cylinder misfire. Codes were erased 2 days ago to see if report would repeat - so far nothing - but engine still needs to turn over multiple times when hot before starting plus, when you open the air filter cover, there is a strong odor of gas. Any thoughts on what might be causing the fuel mixture to be too rich and, is this a possible cause of the starting problem when hot?