Car in question is a 1992 Lexus SC400 with 99 thousand miles, seems to me that there's an intermittent misfire, used to have some shaking but that hasn't been the case at all recently but it still feels like it's down on power, opionions please on if it sounds healthy;
Also changed all 8 spark plugs and this is my biggest concern, seems like a valve cover but I've also read that this could signify low compression...
The new sparkplug after a bit of drving and running, less than 5 miles. Used 8 NGK BKR6EGP's

 

Check for vacuum leaks, replace all vacuum line the are old and dried out. What condition are you spark plug wires, distributer caps and rotors?

 

Haven't thought to check the vacuum lines but definitely will.
And it's got 8 new sparkplugs and the wires are almost in new condition,
How would I diagnose the rotors though?

 

You will have to remove the distributor caps and check the rotors. Look for excess wear on the contact tip and caps. 6mm or 8 mm bolt holds the rotors to the shaft on the distributor.