odd engine performance on 1998 GMC Savana 3500

serpentine belt broke so I replaced it and the altenator had to be replaced also, now the speedometer either reads zero is is erratic and engine acts like it wants to die at around 25-30mph even though my foot is all the way to the floor it doesnt come back to life until it slows down to around 20-25mph and it keeps repeating this.
No evidence of coil or ignition module problems, new cap and rotor and wires and plugs. Does not appear that serpentine caused any other damage to wiring harness or sensors when it broke. What could be causing this?

Asked by for the 1998 GMC Savana 3500
I see your question has been unanswered for a while. I hope your problem has been repaired by now. If it has please let me know what corrected your problem. The only suggestion I can make would be to try and zero in on the speedometer problem. If the PCM is receiving an erratic speed signal that can cause all kinds of driveability issues.
1 more answer
check your catalytic converter