Armed with a new filter, new fuelpump and a handfull of tools I started trying to diagnose the problem. I had allready ruled out the mass airflow meter by diconnecting the plug and taking it for a drive - no change. had to be fuel pressure.
I jacked the car up an put it on axlestands and removed the main filter. The old filter had some degree of rubbish in it but not enough to stop the car running nicely. The new filter was fitted and the car taken for a test run.... No change. Had to be the fuelpump.
Opened the boot and removed the 6 phillip head screws that hold the pump cover, undid the wiring clips and used some PET plastic to release the fuel lines. Knocked the cover undone with a large tang through screwdriver and a mash hammer. I little bit of fiddling and the pump and sender come out as one unit. A quick visual inspection revealed the problem - the 5/16" fuel line between the pump and discharge hose had a big split down it.....
Fitted up the new high pressure hose and threw it all back in. Car pulls cleanly to 6000 now - job done
It isn't always the fuelpump failing.....

So if a 5.0 pings under load then amongst all the usual suspects in the engine bay we can add the fuelpump and fuel filter to the list of possible causes?
Interesting.
Mine pings a little under load and I have never yet to fault the engine for it so fuelpumps and filters is a good thing to know.
Thanks!

Hi guys, back again after lots of trouble getting paid at work for the last few months. Hence no spare cash for spending on the car STRESS!

Thanks for the report Shane, it does sound very familiar! The Ghia is still not a happy car, but is driveable. Hard to start, low power, very lumpy idle, very heavy on fuel, doughy throttle response but no more backfiring through the airbox thankfully.

I'm definitely going to change the fuel pump when possible. I'm still suspecting an intake manifold leak though as the idle is very lumpy, then things smooth out once underway. There is some hesitation when I first crack the throttle and the car won't rev right out like it used to. It only surges slightly under full throttle, but like I said, it ain't making anywhere near the power it used to. No pinging at all.

It must be fuel pressure, but would this be the most evident at idle? The low power is probably a timing issue more than anything, but the ECU could be running it rich to protect the engine I suspect ie. poor fuel economy.

More theories welcomed! It will be nice to get the old girl back to normal again.

Mike