Starting Problems (Wierd?)

Having some small starting issues with my 90 I just purchased not to long ago. I was aware of the issue before I bought it. Heres the deal, most the time is starts fine, but everynow and then it will just crank and crank and crank. If you pop the hood and unplug a sensor (I've been told this is the MAPP sensor, mounts to the firewall sort of over the exhaust manifold shield, has a air hose that runs to the throttle body and a green plug that goes into it) then try and crank it once or twice till it feels like its gonna fire, hook the hose and plug back up and it starts right up first time :confused1. Wierd?? Anyone else had this problem at all or know what it could be? Will try to post pics of the sensor later if needed.

Thanks
SAM