I have a 1995 XJS 4.0

When the engine is warm and I'm at a stop light. I notice the slightest misfire. The best way I can describe it, is every 30 seconds to a minute, I feel a very slight torque on the engine. The RPMs seem to stay constant while it occurs. Obviously no codes.
Recent maintenance in the past year. New spark plugs, O2 sensors, cleaned EGR & TB. Hoses seem in good shape but I havent done a smoke test to rule out vacuum leak.

Where to start with diagnosis without throwing a bunch of parts at it? Aside from the stumble at idle, it seems to run ok and does not ever stall out.

 

My 1996 was doing that and was driving me crazy as I had done all the things you have done. Eventually it started to stall out when warm but would start right back up. I reset the throttle body gap and replaced the crankshaft position sensor and both problems were fixed. I suspect the stumble was the cps and the stalling was the throttle valve closing fully, but not sure. Anyway, I haven't had any problems since.

 

I'll look into the CPS. Doesn't seem like it can be tested without a scope. Is there an aftermarket option that is OK to use? It seems like the only options are the $300 OEM part or $25 chinese junk. I am skeptical about buying a used one.

 

In this case, don't poop on the chinese junk.

I've only bought the $25 chinese junk CPS and it has served me well for 8 years now. I daily drive this car, 12 months a year. I figure that CPS has 80,000 miles on it now. Do not bother with a used one.

With this part being simple to replace, I'm not sure why someone would want to pay $300.... chinese junk can be replaced 12 times in that time frame....