My wife's 2000 Chevrolet Venture minivan will not start. Anyone with pointers would be much appreciated.?

The starter works fine occasionally, so that's not it. A bad starter does not work on occasion. Bad starters usually do not work period. The old battery checked out, but you replaced it anyway. Plus you used your truck battery to try. So having used three good batteries, we know for sure it is not a battery problem. "When I try to crank it, it does not start immediately" "My wife holds the key in start position until it cranks". Based on those comments it is most likely a bad ignition switch on the steering column. $150 for both the part and one hour minimum shop time.