I would love some direction on this one

I have a 1990 XJS and it has been running perfectly for years, it has had a 5 speed conversion, and is a great drive normally.
As it's winter here in NZ and it's been 10 years since I have had the radiator rodded I decided to take it out and get it done, as a "stich in time" as it had not overheated. So quiet a straightforward operation, a process I have done many times before, but once I had reinstalled it, it now has a problem!!
It starts and runs fine and accelerates fine up to about 2000 rpm, after that you can feel it reducing in power until it gets to about 3500rpm (driving, and 4200 if sitting still) and it won't go any higher. It appears to be running on both banks, when you run it flat out and then on over run you get the slightest puff of black smoke but not much. Temperature is half way between C and N when cruising (it is winter here in NZ). Once hot it will fluctuate at idle going from a few hundred rpm to maybe a 1000rpm. From what I have read here I think it's an electrical fault, to me it appears to be running out of spark. Could it be something to do with the ignition amplifiers?
I have checked the grounds on the plate above the radiator and they seem fine, I am going to check all the wiring that we touched at the front for breaks/faults but after that I am a bit lost. Clearly it is something that we disturbed when we took the radiator out but the reality is we didn't disturb too much.

Any suggestions greatly appreciated.

 

1990 = Marelli. Failing amplifiers on the radiator top panel/wiring giving up? They have been moved when you got the rad out.

 

We didn't move them just undid the panel and folded them back over the motor. I agree though I think they might have lost their connection some how.