Idle and running problems

First off we just bought a 90 LS and it had a pinging and bad ticking so went to have valves adjusted. after the valve adjustment it has been running wierd on and off. sometimes when your stopped in drive or in park the idle will bounce down to 200rpm and almost die then fly up to 1100 then fly back down and so on and so on only does it about 10 times then stops didnt do that before valve adjustment, so we just had the valves adjusted again today and still does it but it runs better.
second when decelerating it will try to cut out and it will studder but rpms dont drop it is like brakes grabbing and letting go softly but its not brakes.
And this car has just no balls, we had one before this same year and model that was a lot faster, both same mods just a cold air intake and 8mm ngk wires… any ideas?? same mileage on both about 110k and the first one was hot rodded on and this one was a grandma car…
Thanks for any help…

Any help would be awsome.