The good news is by having the Startup text duplicated, your Video seems to be working. I'd think something is causing it to continually
try to go through the boot up process in a loop, making the duplicated lines of text.
I'd start by printing the GIF of the Power Supply and check each with a O'Scope making sure they are not dropping low causing the reboot cycle.
(+9 Unregulated, +5 Regulated, -5 Regulated, +12 Regulated)
You likely will also want to check Power Supply Sections for Ripple.
http://www.classiccmp.org/dunfield/r/r650x.pdf
http://zimmers.net/anonftp/pub/cbm/schematics/computers/pet/univ2/8032087-01.gif
+5VDC Pin 8
GND Pin 1, Pin 21
Next, I'd be checking the CONTROL BUS Lines going to the 6502 CPU that have PULLUP resistors, which will allow you a quick and easy test
by pulling each Signal (Control Inputs to CPU) LOW. It's likely you could have one of them being toggled LOW continually giving you a RESET
Condition, or an EXTERNAL ROM PACK or such giving the RESET Condition. Signals SO*, NMI*, IRQ*, and RESET* are active LOW, and
should not be pulled LOW (or held LOW) all the time when the Computer is running properly.
Control Section with PULLUP Resistors
SO* Pin 38
RDY Pin 2
NMI* Pin 6
IRQ* Pin 4
RESET* Pin 40
If all the 6502 CPU CONTROL Signals look good, and can be toggled LOW momentarily, I'd build up an IC Socket to exercise ALL the ADDRESS LINES
while you check each with a O'Scope. This will also allow you to check IC's UD13 & UD14 (74LS244) with your O'Scope.
Address Bus
AB0 Pin 9
AB1 Pin 10
AB2 Pin 11
AB3 Pin 12
AB4 Pin 13
AB5 Pin 14
AB6 Pin 15
AB7 Pin 16
AB8 Pin 17
AB9 Pin 18
AB10 Pin 19
AB11 Pin 20
AB12 Pin 22
AB13 Pin 23
AB14 Pin 24
AB15 Pin 25
The NOP Instruction for the 6502 CPU is 0xEA (1110 1010)
D7=1
D6=1
D5=1
D4=0
D3=1
D2=0
D1=1
D0=0
So, if you wire an IC Socket so the PINS for all DATA Lines to the CPU (NOT Connected to Motherboard) are STRAPPED such that the CPU
continually gets an 0xEA Instruction, the CPU will Increment the Program Counter, and cycle through all the possible addresses. Just insert the
IC Socket in the Motherboard, then insert the 6502 CPU into the IC socket. Just make sure none of the DATA Lines on the Motherboard
are connected to the 6502 CPU. This will allow you to attach your O'Scope, and check just about everything/anything connected to the
Address Bus (ie. UD13 & UD14 74LS244) by chasing signals. (You could also take an old, good, spare 6502 CPU, bend the Data Bus Pins straight
out, and wire wrap those for an 0xEA. Then plug the unmodified IC socket into the Motherboard with the modified 6502 CPU inserted into the IC Socket.)
This ONLY leaves a few Misc Control Lines unchecked, which aren't likely to be your problem.
Control Section Misc
NC Pin 5 (NO ROM*)
SYNC Pin 7
Phase 01 Pin 3 (not used)
Phase 02 Pin 39
R/W Pin 34
16 MHZ Pin 35
Phase 00 Pin 37
So you will have checked 90% of the CPU and associated circuit, with the exception of the Data Bus Lines, which could be another source of the
problem. To check these you will need a Logic Analyzer or a Monitor ROM that allows you to single step through a bit of code to place data on the
bus and chase signals, or read from a specified location, and verifying the data is proper.
Good Luck.
Larry