As a percentage, how many have your zigzags gone? If you can get a hand in, check that the remaining zigzags are actually attached - sometimes they are there, but no longer bonded. Ours looked a bit like that, but as it came out, pretty much all of the zigzags fell off.
If you have lost the zigzag fins in appreciable quantities, then replacing the rad is a good idea anyway.
The problem in diagnosis that that it is only in trouble at speed.
It can be one of two things: inability to transfer the heat out (knackered rad), or a flow problem (blockage in the rad, knackered pump, duff thermostat not opening fully). I've never seen a thermostat fail "closed", they always seem to fail open, by design.
Do you know the history of the car - has it always been run with decent antifreeze? If not, then the rad is probably full of clag as well as losing fins. It needs changing every few years.
Some ideas to further investigate:
- If you've got visible problems with 10% of the fins, then the rest are probably shot - replace the rad, it needs doing anyway.
- Try reverse flushing the rad. Disconnect bottom and top hose, stuff a hose pipe in the bottom and run a load of water in. If loads of goop comes out - replace the rad.
If your rad passes those tests, then you're into rather annoying territory of considering the pump and thermostat, but my money would still be on the rad. The system is fundamentally OK as the car does not overheat in traffic or normal driving.