Do I need a stronger wheel?

Thanks, mate. Suggestions on stiffer rims that aren't exorbitantly expensive?

People on here will advise Deep V's etc. But I'm not a fan of gaining stiffness through depth alone. Its not like you see many touring bikes designed for heavy loads, rocking H and Sons hipster rims.

I'd go with a strong eyeleted rim with 32/36 standard butted spokes (Sapim Race, DT competition, etc).

There's loads of touring rims around. Some examples.

Velocity dyad
Mavic A319
Mavic A719
Halo ExciteR
DT Swiis DK540

All the above rims are wider than a standard road rim, so will only be compatible with 25mm or larger tyres. But a 25mm tyre on a 24mm wide rim, will roll nicely, and will be less likely to pinch flat, then one on a narrower rim.

I have recently built some new commuter/load carrying wheels with the Halo ExciteR's and really liked them. They where really easy to build with, and the wheel feel nice and stiff, even when the bike is loaded.