I have a Ford Ranger Extended V6 3.0 4x2 with an automatic transmission. The truck has 150,000 miles on it, but I had a brand new engine installed about 30,000 miles ago.

I am having two distinct issues, though I believe they may be related.

First: At high speeds, 80-90mph, it starts to lurch. It feels like it's trying to leap-frog forward. It almost feels like it's trying to shift, but unable to get into gear, so it falls back and tries again.

The second issue: When at idle, the engine proceeds to rev up then slow again. It does this more severely when in drive than in park. When in park, it bounces between 800 and 1000rpm. When in drive, the idle drops, causing it to bounce between 550 and 1000rpm.

What do you guys think? Any help would be appreciated.