running problems '97 adr 1.8 A4

Greetings
I have a problem with my A4


1. Year of car                              1997
2. Model of car                            A4 1.8
3. What engine & TIP or 6sp           ADR
4. Miles                                      car = 140k engine = 85k (cam belt tensioner bolt snapped @ 120,500miles (500 miles after                                         120k service) fitted used replacement ADR engine (mileage on  replacement engine70k)
5. FWD or Quattro                        fwd
6. Any CELs or stored codes           (not that i know of)

this is what happens

start car (engine starts as normal)
but stops straight away
start car (with a tiny dab on gas pedal) to keep it running, take foot off gas and revs drop quickly and stops
engine sounds normal
drive off , for about 15mins need to dab gas if stationary or revs drop and engine stops
after 15mins (roughly) when clutch is applied, revs go up  and fluctuates from 3,000 to 4,000rpm (up then down , up then down)

what i have done

air filter         (new)
airflow meter  (new)
throttle body  (clean)
injectors        (clean)
plugs            (new)
oil                (new)
oil filter         (new)
adjusted throttle cable to its max (in an effort to keep engine running) (not a fix i know but this has started to do my head in lol)

so, thats my car, thats what it does, and thats what i have done.
If anyone has any suggestions on how i can fix this problem
i would be most gratefull

Kind regards

Tony

Go to AutoZone and have them check for hidden codes.