Probably a cable problem. They tend to bind more at lower speeds and smooth out a bit as you speed up. Try pulling your instrument panel and pulling the inner speedo cable out. Wipe it clean and then reinsert it as you squirt some graphite on it (don't use oil). the inner cable should slide right back in if you twist it a bit as you reinsert it. If the cable is in good shape that should solve your problem.

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LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold