Isuzu Starting Problems 99' Rodeo

I have a 99 Isuzu Rodeo, 4wd, 6 cyl, LSE, Auto. It has 60k miles on it. I just got through replacing spark plugs, new timing belt, new water pump, new intake gasket, new fuel sending unit, new cooling unit (measures temp of vehicle and somehow affects the fuel system), new fuel filter, new air filter, oil change. The problem is still there. I bought the suv used about 53k miles, and the problem has gotten worse. It will not start right up, it cranks for about 5-10 seconds before it turns over and starts, then it take about 20 seconds for the rpms to level out at normal idle. It also stalls out sometimes at low speeds, less than 3mph or when shifting from fwd to R. I saw that there may be a fuel pressure regulator on this vehicle, but I don't know if that could be it. If anyone out there thinks they may be able to help me I would really appreciate it. I have taken it to the shop and had it put on a computer, and they told me that everything checked out ok. The computer is definitely missing something. Please e-mail me if you can help me with my problem. snipped-for-privacy@charter.net Shane