Jolly_roger_uk

I'd say it's a radio issue. Killswitch is killing the engine due to signal loss,  as it's the same distance each time.  Then as you get close to the model, it has picked up radio signal again.
Are batteries in the transmitter and receiver charged properly?

The antenna on the receiver might have been damaged, can you check that?

You could check to see if it is a radio issue, Flysky GT3B?.  Adjust the end point for the throttle and turn it down but remember original number, so the car doesn't fully rev up, but it revs enough to hear it 30ft out. Sit the model on a box, or take both rear wheels off and sit it on them (so the model can't run away, wheels off the ground). Disconnect the killswitch on the engine side, or if it has a connector inbetween use that. With the model running, walk away while blipping the throttle every second, and see if the model starts to lag or respond differently as you're walking further away. If the throttle sticks open, or doesn't respond, it could well be a radio issue. Might be worth having someone next to the model, incase it decides to go full throttle, so it can be turned off via the kill button and not cause any unwanted wear to the engine.

Using the GT3B, my model starts to get lag at around 80ish meters out (best guess, been a while), but doesn't cut out.

Thanks Vr5fx 

Really appreciate the advice - I’ve fully charged the receiver batteries (green light on the charger) but didn’t do the ‘charge for 4 hours the first time’ as suggested in the manual. It turned green after an hour or so I pulled the plug, I didn’t want to over charge it.
Batteries in the transmitter were fresh AAs.

Antenna seems fine visually, no issues I can see. 

Yes Flysky GT3B. Will have a look at the endpoint but pretty sure I’ve p*ssed up the factory settings. No idea what they were at right out of the box. 
 

Thank you for the radio testing suggestion - I’ll have a go during the week and report back. 
 

Really appreciate you taking the time to respond too!