1993 Plymouth Voyager Engine stalls soon after starting, bu

Hello guys, hope you can help me with this. My 93 Grand Voyager 3.3 liter starts up fine, then within 20 seconds, I hear a click from the fuel pump relay. After that, the engine runs rougher and soon stalls. It restarts right away, but when I drive it, the engine intermittently bucks and surges, usually when it is about to shift or when climbing a slight hill. When this occurs, the speedometer will drop down to zero and then bounce back up. It runs fine at high speed on the highway and seems a little better overall once the engine has warmed up. I tried swapping the fuel pump relay with the adjacent one on the relay bank, but with no change. My mechanic told me it sounds like the transmission speed sensors were bad. I took them out but didn't test them, but did make sure the connections were ok.