98 Blazer Erratic Tachometer???? NEED ANSWER

Since Feb 04 when I bought my blazer I have managed to sink just over $1000 into it in steering/suspention parts, overheating, alternator and 4wd/tranny problems! I cannot seem to figure out why my tach needle jumps all around.

I am driveing normal down the road and the tach needle will suddenly spike or bounce around for a second then seddle back down where it belongs. It dosent matter the weather conditions or anything. While under a harder acceleration it will actually go from reading 3000rpm to bounding around very fast at the 5 and 6k marks erraticly and sometimes u can actually feel the truck hessitate with the way the needle is jumping and seem not to shift correctly/slip a little with the needle....I am lost, any help would be great!

Does the motor react with the tach? Or does it go seperate? It may just be an issue with the gauge if it goes seperate.

9 times outta 10 the engine will react with the needle jumping. you will feel a little hesitation/skip when it does it, especially under a harder acceleration

No SES? It may be a sticky throttle or something wrong with the throttle in general. Wish i had a pair of diagnostic dice.

The ses light has never been on