I have a cargo van 2003, 6 cilynders 4.3 lt, some times run like have misfire,

sounds like you threw alot of good money at it,try taking it to a good ASE certified technician and pay for some time to get it diagnosed,it might be more cost effective

The days of DIY auto repair are over!!.. As suggested, it needs to be tested to find the problem and then fixed.. Good mechanic will perform a fuel pressure test with a full scan test which includes viewing live data to help pinpoint the trouble, can't guess at it ... Cost too much to do so!!