Tachometer and Speedometer Not Working

I know very little about vehicles. My question, is at times my tachometer and speedometer will not work on a 1996 GMC Yukon 4x4. It does this periodically
and with, I think, no consistency to weather as I live in the cold northeast. The worst part of my concern is when this does happen the vehicle takes about 5-10 seconds to get up to speed with the throttle pushed down approximately halfway to more. Even then the truck stays in third gear at/around 2200 RPM's. I am hoping to get some sound advice before taking it to a mechanic. Any help is greatly appreciated, thanks in advance.
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Does the speedometer ever read incorrectly or does it just flat line?
When does the speedometer stop working? Doesn't work at all when you start the vehicle? After you've driven for a while? What about the tach?
Does it not work when the vehicle is cold or after you've done a few trips?
How old is your battery?
Steve
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Thanks for you response Steve. No, the speedometer never reads incorrectly. When they are inoperatable - both the speedometer and tachometer do not work. I can say it's more commonly when I first start the vehicle that I am alerted of the needles but sometimes after some driving they will return to normal. The battery is less than 3 years old. But most importantly do you know why the truck would be so difficult to get up to speed? Is it the transmission or something worse? I just don't know how it can all be related. It's like when you're moving away from the "No Wake" area on a lake and the boat is slowly motoring along, then when you slowly give it throttle it will pick up. If that makes any sense. Hopefully you have some advice. Thanks again Steve.
Ron
Steve Mackie wrote:

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Any warning lights on at any time?
Steve
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Yes....the service engine light and the ABS light as well.
Ron
Steve Mackie wrote:

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Have the codes read, reset the light(s), test drive, have the codes read if the light(s) come back on, report back your findings.
Steve
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