Series 2 - will NOT change channel at first

[dmband]

i have had my series 2 for a few yrs and it has worked fine
recently however it would not change the channel on the cable box for approx 3-4 mins of turning the tv on
~ tv sits with the channel unchanged for a while - ie overnight
turn on tv and the channel will not change on the cable box even though it changes on the tivo and shows the change on the tv screen
~ tv sits for a while and channel will change on the cable box UNTIL you turn the tv on then it stops changing even though you see the channel change on the screen

i took the cable box back and got a new one which worked 100% for about 3 weeks and now is doing the same thing

i am totally confused because you can see the tivo changed channels
you can go into the tivo menu where you can view recordings and everything else
you can use the cable box remote to change channels and the correct channel shows on the tv
it also doesn't make sense to me that the tivo and cable box will change channels with thew tv off but as soon as i turn the tv on it stops working correct for a few minutes

any ideas?
thanks a lot