"muffled thump"  Could be you have a thrown belt on the balance shafts.
Trully mysterious.
Pull the cover off the timing belts and see if everything is in place.
If you have over 40000 miles on the belts you need to change them.
The cam belt may have jumped a few notches and threw the valves out of
timing.  This would make the engine run stangely.

Firstly, very many thanks to all who offered suggestions as to why my late
'85 944 Lux was spluttering & losing all power intermittently ( see my post
1 August ).

Since then, I've replaced the DME relay, the ignition coil, the fuel pump
and the fuel pressure regulator, all to no avail. It's had a new plugs,
dizzy cap & rotor arm recently too. The problem first arose when I was
driving at about 60mph - there was a muffled thump noise and the car died
totally just for a second, then picked up again. It keeps doing this, but
there is no pattern apart from it seems to do it a lot more going uphill.

Please - has anyone got any more ideas, as I am tearing my hair out now in
frustration !!!!!

Cheers folks.

Myles