hard to start cold on 1998 GMC 1500 Pickup

4.3 v6 hard to start when cold starts fine hot has missfire around 2300 &5000 rpm

2 answers
From previous experience there is a TSB (Technical Service Bullitin) that maybe the problem.It concerns the crankshaft sensor at times can cause ECM to set timing to 50 degrees advance causing starting problems.
Check the fuel pressure of your 1998 GMC 1500 Pickup. It should be 60-66psi with the pump running and the engine off.