[olopezm]

Also check the vacuum hose running from below the IAC valve to the fuel pressure regulator and to the transmission modulator. You can easily see the modulator if you look down from the upper radiator hose. If it is loose or cracked/roted it can develop vacuum leaks and cause shifting problems.

How long since you last replaced the transmission fluid, the filter may be getting blocked and that can cause slipping too.

Best regads,

Oscar.