Without looking at it I would think its a boost leak probly from the intercooler. I had a similar problem. My mate took off the undertray and pushed the pipe back on and iv never had the issue again. Leak has to be the door seals as they are rubbish on mk1 and there's loads of threads on here about it so hopefully that should solve that one. Hope this helps. Doing stuff to a car isn't hard mate just the idea of doing it wrong scares people. Just remember to put back on what you take off .
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Cheers you're right it's not the actual doing things that's probably difficult it's getting things wrong that worries me. I tried fitting a deck spacer to my old Celica around a year ago and ended up dropping one of the inlet manifold bolts down into the manifold which was then followed by not being able to use the car for a fair while lol
Thanks again