Problems With Gear Changes And Engine Performance

HI guys,

New to this site, having some major problems with my partners 1.25 L Petrol fiesta '04 model ( i drive transit). Always had ford vehicles but having a nightmare with the missus' car. Had the exhaust changed to pass MOT about 6months ago, and the day it got done I said to the wife....''you know that sounds crap, sounds like its blowing''. took it back to the garage got the all clear from the mechanic (who is a friend). But performance of car is now terrible but seems to be more of a transmission problem. Gears 1 and 2 engage ok, try and get it in 3rd and you have to wrestle it in and it just judders and loses power, problem gets even worse when changing down gears (all gears) and the engine feels like it going to completely cut out sometimes not even letting you select a gear until road speed (not revs) reduces. Could this be clutch not engaging/disengaging properly? we have had previous problems with the master cylinder on clutch pedal but fixed. could hydraulic pressure not be adequate?

We also get a very loud knock and rattle from engine bay when pulling off drive at 2/3mph off small kerb then the same engine judder.

All these problems have come about since the ''new'' exhaust was fitted, fuel consumption is worse,getting crackle and pop of exhaust when revs run down (like un-burnt fuel in manifold/exhaust) this makes me think could it be the exhaust or injector problem, with the blowing noise problem and lack of performance on gear change Im hoping that it is exhaust not fitted properly. But the problem with gear changes up and down is worrying that it might be gearbox/transmission problem (hopefully just clutch master cylinder on pedal).

There are no engine management lights showing and because the problem is intermittent (although getting more and more regular ie more often than not) mechanics cant find fault with it until it actually dies on us.

I do have a basic knowledge of car maintenance and servicing but this one is beyond me as I've said above what I think it is, but was just wondering if anyone has had this problem or what their thoughts are on it.

would be great to hear you thoughts guys,

cheers Daz