94 Bonneville shakes after spark plug change

I have a 94 Pontiac Bonneville with a recently fixed head gasket (and heater core, that caused the overheat). It was running fine for weeks. I thought I'd make it run that much better and tossed some new plugs in it. As soon as I did, it now shakes at 55 mph if gas is adjusted slightly. I have to either let off, or gun it to stop the shaking. I double checked the gap and replaced the wires, still does it.
I've done research and so many ideas come. EGR Valve, throttle control. I've double checked to make sure I didn't unplug any hoses during the change.

Can anyone help me from buying/fixing part after unecessary part.
Thanks

The first thing I would do is to double check the wires to insure they are all on the right plugs. Since the car only has the problem around 55 then that probably is not the problem. The other area to check is the CV joints. They will show up as a high speed vibration at around 55 to 60 mph and if they are bad enough they will cause a shaking of the entire car.

I've got an 94 Olds 88, which is the exact same thing as a bonneville, just a different body. Anyway, I had the EXACT same problem a while ago. The problem for me ended up being bad spark plug wires. I had done some research and it seems that this happened to quite a few people. If it isn't your wires, you might want to check your coil pack. An easy way to see which wire/coil pack is bad is to take a spare plug (if you have one) and pull the wire off your spark plug in your car and plug it into the spare. Turn your car on and bring the spare plug to something metal on your car. If the plug zaps the metal, then that wire/coil pack is good. Do this with every wire. If you don't have a spare plug laying around, you can use pretty much anything that is metal and can be plugged into the wire. If all your wires/coil packs are good, then it could be that you got a bad spark plug, or one of million other things that I don't know about.