Toyota Corona 3S-FE problem

Long story short. After first indicators, two break downs with tows, two visits to the mechanic ($445) with assurances at each visit thatthe problem has been remedied.
The damn thing began stumbling again today after 7kms travel. We managed to reach our destination without coming to a complete stop ; a distance of 12km.
Looking back, this 7kms seems to bethe common distance when the problem first shows, then gets worse till loss of ignition.
Anyhow, on our return, sure enough at about 7kms it begins to stumble etc. We made it home.
Now here is the interesting bit. Sitting in the driveway we ran it up the rev scale and held it there until it began to miss(!) and run rough. I smelled something that seemed familiar so into my chair and to the exhaust end.
Sure smells like burnt fibreglass (!). I got my wife to drop the revs and I felt the pressure at the pipe- nottoo strong I thought. So she brought it up to higher revs and I couldn't detect any appreciable increase in flow but it was bloody hot and that strange stink.

Here is the thing; would a partially blocked muffler cause the car to behave in such a manner by effecting the engine management system!
Items replaced by mechanic: fuel filter, distributor cap, rotor, electronic coil, plugs.

Can anyone shed light on this! I am tired of throwing funds at it with no result.

probably blocked cat

Gareth Mogan will be happy!

On a serious note I'm inldined to agree its an exhaust problem but if the cat is blocked would that not throw a code and put the check light on!

What year is this car, NZ new!

blocked cat or stuffed rear muffler causing it to choke also when was the last time the plugs and leads where done

I would start by testing the coolant temperature sensor and checking for air leaks around the intake manifold. If those are OK, I would move onto checking for blocked exhaust with a vacuum gauge.

Thanks for the early responses. I'm from the old school of carbs, dizzys and ballast coils and I'm not up on this electronic stuff.
It's 1993 NZ new so non-cat. Manual and has 245k on the clock. The mechanic replaced the plugs ( I presumed that he checked the leads for resistance. We have owned it for 6 years and I cannot recall if I changed the leads way back).
That smell from the exhaust; I remember it from way back when my MK3 Zephyr blocked- that hissed like hell and was down on power. The Corona is not hissing and as for power it seems Ok, initially:-) .

Originally it took 30km before faltering occasionally then clearing, and she's all good.The first major stop was when the wife took it up to the Kumeu show (3 hours on the side of the road) Then the next day after the first visit to the mechanic it stopped just north of the Orams Brige on the Sth motorway- towed back to the mechanic.Now, once rested it drives alright for 6 or 7km and then it begins so if my suspicion is correct I suspect that distance will become less as the choking becomes more acute.
How do I go about testing the coolant temp sensor! Checking for vacum leaks with spray start type product!I have a vacuum gauge so will look for a vacuum port on the intake