Car won't really accelerate past 30

04 Buick le sabre. Couple days ago it turned off on
the freeway, the word volt popped up on the dash.
Pulled to the side of the road. Started back up but it
started to shake and drive rough. It still accelerated
past 30. Still went past 90-100.however during
acceleration the check engine light blinks when it
goes past 30.  Autozone tells me it needs a tune up.
Couple days later , I get the full tune up, plugs,
wires , and filter. Guy fixing finds out one of the coils
is working.  We go get another coil. And it still isn't
working. So I suggested if it was the ignition control
module.  And he says it could be. But I don't have
the money for it til next payday. Could this be the
problem to why I'm not getting over 30. Oh and btw
it goes over 40-50 and so on eventually. But it only
does this when I let off the gas and tap on it lightly
then let off of it and start tapping on the gas again.
So back and forth doing that gets it over 50. But I
can't do something like that getting on to the
freeway. Helppp please