A loss of power

I know and heard that there could be a number of things making this problem. But a while back like 5 months ago, I was running off of 4 cylinders because my 1&3 coil pack was useless. So my alley mechanic changed it and it was running realllllll smooth. But now it doesn't feel like I'm running off 4 cylinders but its like there a little loss of power. should I changed the ignition module and coils or the exuahst?

we have a 96 bonniville srs and it is losing power also
you will go 60 down a highway and all of a sudden just slow down to about 45mph. we are older people and darn that upsets us some
we dont know the problem either does anyone
please let us know