K90 Controller described a failed hand off involving a VFR aircraft entering PVD's airspace; the reporter claiming the K90 ASR-8 RADAR has been dropping targets for some time and needs repair.

Narrative

A C208 was receiving VFR advisories from East to the West at approximately 4;500 FT. The aircraft went into coast several times. The location of the coast was approximately the FMH 170R 15 DME through the FMH 230R 25 DME. This is a common occurrence. Normally; I flash aircraft to the maintenance scope (E) to ensure they do not drop off. With this aircraft; I did not. PVD approach called on the shout line and stated 'C208 is RADAR.' I looked over to the airspace boundary line and C208 was in 'Coast (CST).' The aircraft was about over the FALMA intersection. I am not sure if the data tag drop off and re-acquired; or if it had been in coast for a while and the updates of the position were sporadic. Moments after the PVD call; the target re-acquired and I flashed the data tag at PVD and they accepted the hand-off. This occurred after the VFR aircraft had already entered into PVD airspace. I did not call the aircraft RADAR contact lost. I believe the primary target existed the entire time. The K90 ASR-8 has been dropping targets for months/years. This same situation very well could have happened with an IFR aircraft; as this is approximately the same location/altitude the aircraft would have been at had the aircraft been on an IFR flight plan. Recommendation; fix the ASR-8.