Try the following:
Boot without startup-sequence. At the DOS prompt enter the following commands:
cd devs/monitors
rename #? :storage/monitors
cd :prefs/env-archive/sys
delete screenmode.prefs
delete overscan.prefs
Now wait for the HDD to finish its work and a few seconds more, then reboot.
You should now see the normal Workbench on your TV.
Open the Devs/Monitors drawer as well as the Storage/Monitors drawer. Drag the icons labelled Multiscan and VGAOnly to Devs/Monitors and reboot.
Now run Prefs/Screenmode and choose Multiscan:Productivity 640x480. Click on Save. The TV should go crazy now, again. Better disconnect it to avoid damage.
Connect the Amiga to your PC monitor. It should display the Workbench with a resolution of 640x480.
Bye,
Thomas