Tachometer & Check Engine Light issue...

[Nilreb]

Ok, so a month or so ago the tachy starts flipping out and bouncing all over the place...No loss of power or anything just the tachy spazzing out…Along with this the check engine light goes on…After a week or so of doing this the tachy just starting sitting at 50% of what it should be for any level of rev, including idle…Mechanic says the computer reads "multiple piston misfire" and his guess is that it's my backup coil...

Now…When I take long drives (40 min plus) doing at least 70mph plus, after a while the tachy rights itself and the light goes off…And most of the time when this happens the display on the stereo goes wonky and reads like every possible place for it to light up is lit up…

Then when I’m back to in town driving, the tachy goes out again, check engine light back on, and radio back to normal…

So, any ideas other than take the mech's advice and throw a 300$ part at the car and hope it works???

?!?!?!?!?!?!?!? H*e*L*p ?!?!?!?!?!?!?!?

~Nilreb