I would look closely at that large vacuum line that hooks on to the back side of the UIM, in particular, where it connects to the UIM. Did the check engine light come back on again after starting at the pet store?
Next, I would replace the PCV valve and the fuel filter.
Then, the fuel pressure would be a good check. It might be the fuel pump. Whenever I have had a cylinder miss due to a plug wire, I always get a code that indicates the cylinder. It may not be electrical.
Cleaning the MAF sensor may help. I have two 97 Lesabres. About every 3 or 4 months, I have to clean the MAF sensor in one of them. It will cut out and start dying. Cleaning the MAF sensor corrects this for 3 or 4 months.