We have found that intermittent reception as you described is most commonly as a result of multipath interference ('ghosting' in the analog world). Multipath is caused by the signal being received at the antenna by reflected signals (off of wall, roofs, people walking aroung etc.).
The easiest solution is to obtain a highly directional antenna and try that. You want an antenna with a very high front-to-back ratio with good rejection of signals in any direction other that the one it is pointing. The ability to reject multipath is generally more important than the gain of the antenna (although by definition, higher gain indicates higher directionality). Usually impairments such as multipath or impulse noise will cause a decoder to fail to achieve lock long before a low signal strength will.
Another note is that if the fundamental problem is due to multipath, placing the same antenna outside and higher will not cure the problem if the multipath condition still exists at the new location. In fact the problem can be exacerbated due to the loss introduced by the longer coax run. For example at the WIBW frequency, 150 feet of RG6 coax will reduce the signal available by almost 6 times (7.5 dB). Often you can get better results by using a directional antenna at the same location. Of course, the best of all worlds is a directional antenna outside at a high elevation.
The ultimate solution to all of these issues, is to let your cable company know that you expect it to make available the local DTV channels without degradation. Same resolution, same multiplex of programs. Let them deal with the antenna issues.
Here in northern Illinois at my house I use an antenna called a 'Silver Sensor' <<a href="http://www.silversensor.com
> which is an indoor antenna that is highly directional, but with modest gain. It is 6 feet off the ground, on top of the TV, in an interior room. I have a solid lock on all stations in Madison, WI (58 miles) without any trouble. The stations there are between 100kW and 380kW ERP.
Brad, thanks for the kind words.