Bentley Continental R rough running

I have a 1992 Bentley Continental R. She spent 24 years in Japan and clocked up 48,000kms over that time. I brought her to NZ December 2016. Cosmetically she was in good shape but I have spent $30,000 on mechanics, brakes, suspension, etc. The mechanical fuel head has been reconditioned as were the injectors.
However, she is not running smoothly.
At idle she surges between 650 and 700 rpm every 3 seconds or so. Under full acceleration she runs fine. At 100km/h (1800 rpm) you can still sense the surging. At around 2100rpm the engine hiccups/misses when maintaining the rpm. Under acceleration the hiccups go away and then when it levels out at at a higher rpm (say 2400rpm) the hiccups start again.
I am wondering whether there is a problem with the Mass Air Flow Sensor and would appreciate any thoughts.
Regards
Ian