having just recently purchased a kr magna 277,00klm i am experiencing a couple of problems. have recently put a 2.5in exhaust on which i'm sorta happy with, car does go better though. however cold start she revs up to 2,000rpm and eventualy drops to about 800rpm but idle is rough. also occasionally she will idle at 1500 revs but must hit a rev limiter or something as it starts to missfire like f1 cars down to 800 rpm then will stop revs will rise and same thing happens for 3 or more times. this has started since i put the sports exhaust on, i also have k&n panel filter.
I'm just wondering if ihave too much air flow and maybe put another smaller hot dog for more back pressure. your thoughts please. also my gearbox seems like its double shifting when changing up. going to check with mechanic on week end. i just hope these are not the start of something bad.

found cracked hose replaced it, idle now ok. cam belt out 1 tooth on rear bank now fixed, timing belt etc replaced. gearbox to be checked on weekend.