ford focus mk1 1.6 petrol 2004

hi had this issue for a while:

only when the engine has got to temperature does this happen....when i am driving and the car slows to a near stop with or without braking, sometimes the revs will drop a little too low and there is a rumble before the revs rise again. it doesnt do it all the time, about 4 days out of 7 id say.

it only happens when the engine gets to operating temp,
it does it only when slowing with our without braking,
thought it was a faulty sensor (maybe crankshaft) as it was intermittent,
cannot find vacuum leaks and or problem with exhaust,
have changed idle control valve and throttle position sensor,
have done an ecu reset after i changed the idle valve,
have checked for error codes and none came up.

any ideas? the only thing i can think of is to check the tps in case it is set too low.

The mk1 had to have a software update to stop the under revving issue where the idle dropped when stopping it slowing down had to get it done on my old 1.8 mk1 since you've tried everything else it could be the isdue