I worked for a diesel specialist for 5 yrs & lost count how many mondies,trannies & jags we had in with simlar faults
the only downside is it could be a few things
sounds like fuel pressure is getting too high tho & going into limp home mode
could be fuel pressure reg
injectors or poss pump
to do a crude leak off check on injectors
you take the leak off pipe off all 4 injectors (hold on by horse shoe type clips)
After the last 1 (cyl 4)
Crimp it but not too tight with mole grips etc
take the multi plugs off injectors
blow the diesel out each injector at top where leak off pipe was
then put ignition on for 5 seconds check for fuel at injector top ( leak off return )
Shouldnt be any or very little.
Then crank engine for 3-5 secs check again if 1 or 2 have lots & others have little diesel showing
the ones with lots are bad injectors
as for the gunk ya found in the filter
it sugests its been ran on dodgy fuel to me as water in the fuel causes the jelly stuff you found
id be tempted to keep checking filter & if it gunks up.again id drop the tank & steam clean it out as it will be sludged up
( common if its been run on red or home brew bio)
As your car keeps cutting out it should flag up fault codes
it might be worth taking it to a diagnostic centre
they will put the scan tool on it & drive it why conected to read the fuel pressure under load
check each injector
rail pressure etc
should be able.to give you answers from that if there any.good
if your local to nottingham
take it to gee diesel services
they will bottom the prob in no time