2008 Mazda 3 with a 2.0L
 

My Mazda 3 runs good when on cold idle but once it warms up a bit it starts to surge or misfire when driving it just jerks back n forth a lot until it gets up to 4000 rpm and it runs fine until u slow down then it starts jerking and running like crap I've had it to a couple mechanics they don't know what's wrong no codes come up on the computer the check engine light is off all spark plugs seem to have spark they say the catalytic converter isn't plugged and changed the transmission fluid but nothing helps.. anyone who may know what wrong or got any ideas would be greatly appreciated

Step one, compression test; better still, a cylinder leak down test. Why? I suspect burned intake valves.

I'm sorry if this sounds stupid but I'm not a mechanic.. but what's a cylinder leak down test?

A cylinder leak down test is kinda-sorta a super-duper compression test on steroids:

• https://en.wikipedia.org/wiki/Leak-down_tester

When used on an engine, it can tell you whether you have a bad head gasket and/or cracked head/block (bubbles in the coolant), an exhaust valve leak (feed pressure leaking out the exhaust pipe, or intake valve leak (feed pressure leaking out the intake manifold.

Vacuum leak
 

That could be a vacuum leak. Start by looking for anything obvious in the engine bay. Could be an actual vacuum hose undone, or could be a leaking hose due to dry-rotting. May be worth looking up a vacuum line diagram as well. Plus, this is a free place to start. Leak-down test is not.