86 gti cab cutting out

Hi
My 86 gti cab is having real issues …
Recently fitted an immobiliser and since then it hasn't run well
The tick over slowly rises to 1800 rpm and stays there …
After 20 mins of running the main pump starts buzzing more than normal and then the car cuts out … will run again but only after about half an hour then repeat 

Changed the main pump but still the same 

Help 

Do you have a loud whine from the drivers side rear wheel arch?

If you answered yes then the in-tank pump is usually toast and needs to be replaced…

Things happen and it may not even be tied to your immobilizer.

Thanks for your input… yes the main pump was making a loud noise so changed … but still has the issue
I guess you are right in that it is under strain due to the faulty in tank pump