I have a 351w in my 89. It is a Jasper motor and I just had it installed. The mechanic installed a new throttle cable because it was idling high; this seemed to fix it for a while. I drove it to AZ and it was doing fine till today. Now if it is in neutral it idles around 2k rpm. In gear it idles at 1k rpm. When it wasn't idling high it was around 500 rpm. Any ideas as to what is causing this?