f300 1992 runs rough


Feroza 92
over 3000 revs and usually runs Ok maybe a little under powered. But around just above idle it splutters at times New plugs/distributor cap no change. Jet blasted clean the engine and it ran well for half an hour then returned to missing at low revs say around 2000.
when it gets really bad a quick rev to 5k in neutral seems to fix it for a while.

Is there a diagnostic tool to connect? I have OBD but 92 is too old it seems, maybe a converter?
Ive ordered a new coil but have a feeling it might be a sensor or a problem with the distributor vaccuum advance.
Dont know really but any help will be much appreciated