  The evidence for refractive interstellar scintillation (RISS) being the main
cause for rapid intraday variations (Intraday Variability, IDV) in Quasars and
BL Lacs has recently become stronger. If IDV is still a complex composition of
extrinsic and source intrinsic effects, the intrinsic part of the IDV pattern
should show up in the millimeter and sub-millimeter regime due to the frequency
dependence of RISS. Hence, observations at higher frequencies are essential in
order to exclude RISS as the sole cause of IDV. Here we report on our new
attempt to search for rapid variations at much higher frequencies. In addition,
the possibility of a direct detection of the postulated scattering screen in
front of IDV sources will be discussed. Our recent line observations towards a
few IDV sources lead to the first detection of a high latitude molecular cloud
in front of an intraday variable radio core.
