crazy tach

[1990laserrsred]

alright please help me out
i just finished a year rebuliding/restyling project on my 90 laser
everything is good to go except my tachometer is acting crazy at low acceleration in 1st or 3nd and when slowing down to a stop the tachometer reads correctly, but at any decent acceleration or when driving in 3,4,or 5th the tach jumps between 1000 and 2000, it even starts dropping lower and lower and struggles to keep up as i accelerate. I thought it might be the guage cluster/instrument panel so i went and got 2 from the salvage yard, i popped them in and the same problem continues. I checked the plug in by the intake manifold and it doesn't snap in snuggly, but it connects- is that the problem, what do you guys think?
it's the only thing wrong with my dsm, other than i need a new muffler
thank you

here is a better pic of the tac, it is mounted on the far left screw of the intake manifold- maybe this is my problem? don't know where to mount it, says nothing about it on the dsm technical cd? can si checked, it is a 90 ecu, any more guesses? i'm definetly out o ideas, and ireally sick of my tach not working correctly, can anyone help?omeone help

 

[blackGSX2g]

My tach started jumping on heavy accelration once. It turned out to be the power transistor. Swapped that out with a good one and now its smooth again. If you've got a spare from a junkyard, give that a shot.