I have an 89 Leredo 6 cyl. with automatic transmission. It usually runs great, but once in a while it doesn't want to shift between 2nd and 3rd gears. the RPMs will shoot up, but it won't shift gears. I either have to pull over to a stop, and go again and its fine, or on a busy street sometimes punching the accelerator works. Any ideas on what might be causing this?

Check your throttle position sensor (TPS). My '88 had flaky 3rd-4th shifts. Changing it, fixed the problem.

You can search the forum for TPS and find a lot of posts regarding this.

also speed sensor will cause same problem

saskjeeper, You might want to change your trans oil and filter, it isn't to hard and it might help on your gears changing on those once and a whiles, also add some lucas trans fix it also helps to coat the gears better and keeps the trans oil from breaking down.