'93 Olds 88 Misfiring

Recently my '93 Olds 88 has been misfiring a lot a few minutes after I drive it around. Took it to a shop and had the air and fuel filters replaced, MAF cleaned off, and the fuel injection system cleaned out.
They also recommended replacing the spark plugs and plug wires (which I decided to do myself). I've changed the back 3 wires and all the plugs and noticed it was a little better (though the plugs and wire were long overdue for a replacement).
1. There's a black piece of plastic that holds back the front plug wires...does anybody know how to pull this off? It seems like it should pop off, however I haven't had any luck. Check out Figure 1 on the link provided; it's next to the 4th plug wire (directly to the right)...
http://www.autozone.com/servlet/UiBroker?ForwardPage=/az/cds/en_us/0900823d/80/15/c3/3d/0900823d8015c33d.jsp
2. Even after replacing 1/2 the plug wires and all the spark plugs, I drove it around for a while in the neighborhood and it started misfiring again just as bad as it was before I took it to a mechanic. Any other suggestions as to what might be causing it to act up? So far all I've brainstormed that either the computer has gone haywire, throttle position sensor has gone to junk, or maybe those front plug wires are worse than I thought.
Thanks.
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find where the back plastic is attached to itself. push it in at that point and pry it off. there is a clip like edge on these plastics if i remember it right.
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http://www.autozone.com/servlet/UiBroker?ForwardPage=/az/cds/en_us/0900823d/80/15/c3/3d/0900823d8015c33d.jsp
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First finish relacing the plug wires so you can rule that out. I am not sure what that black piece of plastic is, sorry.
I bet you the misfiring is due to a vacuum leak of some kind. If you follow the vacuum lines there will probably be one that has come off or is cracked. You'll need to find it and replace it.
If you hear a loud hissing while the engine is running you can bet you've got a vacuum leak somewhere.
When you have a vacuum leak, too much air is drawn into the engine for the amount of fuel that it is taking in. The air:fuel ratio will be wrong, and the engine will run lean. A lean running engine will definitely have a misfire.
There's a good chance this is the problem.
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Upon further inspection, that "black piece of plastic" has two clips on the end of it (on the right side if you're looking at the engine) and then it slides off. Semi-pain to get off, but a further pain to get it back on...
Strangely even before changing the front 3 plug wires, the car ran normally with no misfires (however after driving a bit, it started coming back). I changed the 3 wires and things seem to be back to normal. It seems to be running a little bit under powered but I'm either driving it extremely cautiously or maybe something else is running a little below a 100%.
Thanks for the suggestions/help.
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