it does sound like a lean mixture (rather than weak sparks) and the #1 cause of lean mixture as you describe is an air leak into the inlet manifold, letting 'unmetered air' into the engine. On engines with Oxygen sensors and 'closed loop' running this kind of fault can be masked but on 'open loop' cars (most pre-cat ones) this kind of fault results in lean running.
Often the idle mixture can be artificially enriched in some way to compensate for the air leak, but these adjustments may not compensate for an air leak at part throttle. Note that when the throttle butterfly is wide open, there is very little pressure difference between the inlet tract and the outside air, so there is relatively little (unmetered) air drawn in through any given leak path.
So it is best to check for leaks into the inlet manifold; leaky/disconnected vacuum hoses, cracked/leaky rubber intake boots, that kind of thing.
Some connections to the inlet manifold on some cars are basically a controlled leak (eg some cars have a cabin temperature sensor that has air drawn across it all the time, and that air passes into the inlet manifold), but most of the devices on the end of vacuum lines ought to be vacuum tight.
It doesn't always taste nice (tip; connect the hose under test to a clean piece of rubber hose when checking any given line) but you can test these by sucking on the end of the pipe that connects to the inlet manifold and putting your tongue over the end of the hose. Your tongue should 'stick' to the hose (which has a partial vacuum in it because of the sucking; practice on a plugged length of hose to get a feel for it). If there is no suction on your tongue, the device at the other end of the hose is leaking air, and may be the source of your troubles.
Dunno what exactly is fitted to the vacuum system on your car (lots by the look of it) but possible culprits include vacuum brake servo, fuel pressure regulator, 'warm-up' air divertor in the inlet to the air filter, vacuum advance module on the distributor, other vacuum motors.....
FWIW the narrow (nylon?) vacuum hoses seem pretty reliable but if one rubber vacuum hose/joiner has split the others will be well on the way; on a lot of cars these hoses are amongst the first things to fail (I've seen the same thing on Vauxhalls, SAABs, BMWs and Volvos...). I'm tempted to say 'if in doubt, replace the lot'; lengths of rubber hose can be bought for this purpose.
[When I've run out, in a pinch, I've used, er, similar hose from an old washing machine; in fact this seemed to be much better quality than the stuff used on the car...…]
hth
cheers
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