1990 mustang gt manual

I have 1990 mustang gt and I'll be driving and everything is okay but let's say I'm in 5th gear if I go into neutral to coast the rpm rise to 3k, sometimes even stalls. I bought a new iac and fuel filter and I haven't stalled YET but I haven't drove it much , but it's still rising in rpm to 3k...

I've had my white beauty for a while now... a 1990 Mustang GT convertible manual transmission .. she's mainly stock except for the Koni suspensions.. which I installed nearly a decade ago but I feel it needs to be changed. Does anyone know how long do they really last, besides the lifetime...