The CC setting ok but lacks the holding power the set speed is a sympton of a leacking vaccuum hose although slight its there.
Below is a function tests which have carried and which works, my system was playing up for months, some times it would engage and some times it wouldn't, the fault was a tempermental dash mounted master switch although it seems to be behaving its self now.

Anyway with the AMR1173 ECU there is an electrical system function test that can be done, which will show the suspect area/item that is giving the issue.
With the more up todate ECU AMR5700 the testing principal is the same although the termination and wiring colour code is differant.

Any vacuum issues is either with the pump, bellows. brake peddle switch or the vaccum hose used to link those three items.

CRUISE CONTROL AMR1173

Ground Test - From pin 8 (black wire) on the cruise control module plug perform a continuity test to good chassis ground.

Main voltage test - With ignition & cruise control system on, and auto box in 'drive' connect volt meter between pin 8 (black wire) and pin 10 (white & yellow wire) you should have battery voltage if not check fuse.

Secondary voltage test - Connect volt meter between pin 8 (black wire) and pin 9 (Purple & Green wire) you should have battery voltage if not check lock out relay or gear selection switch.

Brake Pedal Test - Connect volt meter between pin 8 (black wire) & pin 5 (green & purple wire). Press the brake pedal, you should get battery voltage.

Steering Wheel Set / Accel Test - Connect volt meter between pin 8 (black wire) & pin 3 (red & white wire). Press the Set / Accel button, you should get battery voltage.

Steering Wheel Res / Decel Test - Connect volt meter between pin 8 (black wire) & pin 4 (blue & white wire). Press the Set / Accel button, you should get battery voltage.

Vacuum Pump Test - Using a jumper wire connect pin 10 (white & yellow wire) to pin 1 (white & blue) this provides power to the pump. Then using a jumper wire connect pin 6 (black & yellow) AND pin 7 (black & red) to pin 8 (black wire). This should energise the pump & solenoid and the bellows should contract.

Speed Sensor Test - Connect a volt meter between pin 8 (black) and pin 11 (yellow & pink), drive the motor around the block, voltage should increase as speed increases.
 
Also check the small hoses (about 4mm) inside the actuator housing. In there you will find a small vac pump & an air relay. The ECU drives both the pump & relay to regulate the pull on the bellows. I had issues with perished pipes & changing these plus the pipes to the pedal sorted mine.

The other thing to check is dry PCB joints on the ECU. There's a common problem one on a capacitor in the middle of the PCB.
 
test A RR-P38 (LRL0086ENG)-075.png
May help
Electronic Troubleshooting Manual RR-P38 (LRL0086ENG)-067.png
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