Tach. drop's to zero while driving 70 ?

What would cause my 1997 buick park ave tachometer to drop down to zero when driving speeds of 50 mph or so ? I can feel car bogging out ,losing power then tach. drops to zero car will almost stall but has never actually died ,if i hold foot steady on gas it keeps losing power if i step on gas it feels like something was cloged and broke through as i pick up speed It feels like it go's into passing gear and runs fine for a while , but the tach won't kick back on intill I turn car off and on again ,
we;ve changed the fuel filter &oil filter so we know it's not that ,the service engine light was on and when it started losing power the ck engine light went haywire blinking & beeping ,i do need a tune up , but would that cause the tach. to drop to zero ? my husband power washed the engine could that of loosen anything that would of caused that to happen , Thankyou ,