I'm aware of a bug whereby often times SDV channels will not tune, then simply using channel up/down will resolve and tune the problem channel. However my issue runs deeper with one particular SDV channel (Science HD) channel 335 in my area. About once/week I cannot tune this channel and using channel up/down doesn't resolve it. The only way for me to resolve it is to start recordings on all four tuners (Premiere XL4) excluding the problem channel, then cancel recordings and re-tune channel 335 which always seems to bring it back. Apparently if a tuner sits on this channel for too long the issue will start again, so forcing my TiVo to record 4 different channels somehow purges the tuners. Has anyone else had this issue?