I own a 1996 mazda protege 1.5l DOHC, well what mazda is left in/on her. i started having that prob around the 100,000 mark which is right on time, the mazda engines were 1) not built for much heat (so not performance) 2) they used the wrong mix for an aluminum head (to soft erodes away to easy) 3) they tried a new concept-ed which has to do with emissions, instead of building an engine and then coming up with a system to clean the pollution discharge, they built "well tried" an engine in which is supposed to be clean from the second you start your car. because the first 5-10min or until normal operating temp is reached the cat wont hit its 700 degree+ range to start converting hydrocarbons, carbon monoxide, and nitrogen, so mazda decided instead they would make a floating type valve when engine is cold. in turn when head erodes twice as much compression loss, and as you read 140psi, getting by cold 100psi, so once you have dumped the big bucks into a head/head rebuild you will end up with a good top half but still have a crap block, mazda also f-ed up on there trans. being a low torque engine and a well pretty much performance style trans it drags on your out put shaft from the crank and in turn strains on you rings which end up widening your cyl walls on one side of each cyl. and how you were thinking it could be a timing belt one or two teeth off is a little out there (not trying to offend anyone) but the protege's came with interference engines 2 teeth off would jamb a valve and 1 tooth would make a nice pinging sound when running.

 

Try checking the IAT (Intake Air Temperature) sensor and replace the MAF sensor