Have you done a full tune up? Including all those little things like ERG valve, PCV valve, oxygen sensors, fuel filters, fuel accumulators and other maintenance items?
You might want to check the owner's manual and any steps you can take to clean and lube items before replacing them.
On my +30 year old collector car I shot a bunch of PB Blaster in the warm-up regulator and similar items with diaphragms and it seemed to help. I also replaced all the vacuum hoses and other parts which wear due to age and heat.
The biggest thing I did was to clean and lube up the distributor, if the engine is the heart of the car, the distributor is the heart of the engine. It idles baby smooth now, like a different car.
Link to post by kach22i:
Distributor service (Clean and lube) real easy without removing the pinion gear! - Page 7 - Pelican Parts Technical BBS