Hard to say for sure, but a couple of things to try.
* usually it is not a good idea to use the pci slot closest to the AGP slot when using an AGP card
* try using a different pci slot for the tuner, and see if you can get an IRQ the is not shared (if using W2K/XP, you can find out which slots are using what IRQ and if there are conflicts, in the Admin section of Control panel, in hardware.