My speedometer

I haven't had the speedometer issue but I did have running problems and replaced the engine coolant temp sender brass 2 wire like probe. This was a very easy part change and the engine runs better and better mpg. Because I read this was a common problem with these engines and because the computers are simple and their is no temp gauge on the dash, there's no indication it's not sending the right temp to the computer. So the computer thinks the engine is cold etc and adjusts the fuel wrong etc. The sender just unscrews with a deep socket and the sender is hard to see but you can follow the wires and feel it's just under the top of the serpentine belt area on the passenger side of the engine. Coolant will start leaking out when removed and don't over tight the replacement. Ebay had the cheapest price for me was under $15. Take care

Also there is a battery power cable inline quick disconnect connector for easy power off resetting the computer for mechanics, just after the battery terminals what was green corroded inside when disconnected, corroded from the nearby battery off gases.This was causing the Buick's power to completely shut off the whole car when driving. The connection terminals were so bad I had to just cut the ends off and add permanent crimp connectors. Also check your engine wiring harnesses to sensors etc especially that run between the serpentine belt and look for any frayed wire insulation or missing plastic conduit loom from years of heat and grease. Repair any damage with electric tape and new loom as exposure may be effecting signals to computer.