73 Sonett III and spark plugs issue

Hi all,
I bought a 1973 Sonett III about 4 months ago but had no time until now to work on it.
The car starts fine, idles well, but after 10 minutes of driving it misses a little bit and this gets worst with time. So today i checked some of the basics starting with spark plugs, plugs wires, distributor.
Distributor and its cap were dusty inside: cleaned all those up.
Spark plugs might have been changed 10 years ago! Deposits of various colors, rust on the outside.... I checked the gaps and they were all over the map: 0.024, 0.028, 0.030 and 0.031. They were Bosh W8DC.
I went to my local auto store and got a set of autolite APP64 double platinum. Set the gap at 0.025 as per the service manual.
The car doesn't start as easily as before, doesn't idle well at all and ends up dying quickly, but seems to be doing ok if I rev it up.
What do you think is going on?


Thanks a bunch,
cheers