Possibly some noise on the tx/rx signals. UARTs are very noise sensitive.
Sounds like you’ve hacked up a great solution 
The quick answer to your noob question is that 3 or 5 wires are sufficient. You can either:
- tx/rx, rx/tx, gnd/gnd - minimal but requires xon/xoff flow control
- tx/rx, rx/tx, gnd/gnd, rts/cts, cts/rts - hardware flow control.
The longer answer to understanding RS232 requires that you understand equipment roles, there are two. Data Terminal Equipment (DTE) and Data Communication Equipment (DCE). Dumb terminals, PCs and routers are generally setup as DTE. Modems, leased lines, xDSL modems are regarded as DCE.
As a general rule the DTE will be fitted with a plug, the DCE is fitted with a socket. A further complication is that the pinout changes depending on DTE v DCE. The concept is that DTE and DCE can be connected using a straight/flat cable i.e. pin 1 -> 1, 2 -> 2 … 25 -> 25
For this to work correctly output pins on the DTE side need to align with input pins on the DCE side. This can make RS232 connections VERY confusing. Also, not all manufacturers produced equipment matching the standard.
Key Pins for 25 pin RS232 (DTE)
1 - frame ground (NB. shield wire in cable, soldered to connector frame)
2 - TxD (O)
3 - RxD (I)
4 - RTS (O)
5 - CTS (I)
6 - DSR (I)
7 - signal ground (NB. isolated from frame ground, pin 7 only)
8 - CD (I)
20 - DTR (O)
22 - RI (I)
Key Pins for 25 pin RS232 (DCE)
1 - frame ground (NB. shield wire in cable, soldered to connector frame)
2 - RxD (I)
3 - TxD (O)
4 - CTS (I)
5 - RTS (O)
6 - DSR (O)
7 - signal ground (NB. isolated from frame ground, pin 7 only)
8 - CD (O)
20 - DTR (I)
22 - RI (O)
Note that the DCE side has more outputs and the DTE has more inputs.
Modem Controls
Dataset Ready (DSR) indicates the modem is ready i.e. modem power is on.
Ring Indicator (RI) is used in dialup modems to indicate an incoming call i.e. phone is ringing.
Data Terminal Ready (DTR) means the DTE side is good to go. This is normally asserted when the terminal software opens the serial port.
Carrier Detect (CD) gets asserted by the modem after connection has been made to the outside world and an agreed protocol has been negotiated e.g. 28800 baud.
None of the modem control signals are required for serial port operations.
From memory, the TV910 has configurable switches that determine flow control and which of the RS232 signals it bothers paying attention to.