Your main problem is the battery mix, not the flash. A speedlight should use four matching AA cells of the same chemistry, capacity, age, and charge state. Mixing old 800mAh Ni-Cd cells with 2500mAh NiMH cells will cause poor performance and unreliable firing.

For flash use, the best choice is usually a matched set of low-self-discharge NiMH AAs. These hold their charge much better while sitting unused, so charging them the day before is less likely to leave you with a dead flash. Standard high-capacity NiMH cells can lose charge faster in storage.

Ni-Cd cells are outdated for most users, especially older worn ones. Alkaline disposables generally recycle the flash more slowly, and disposable lithiums are usually too expensive for regular use.

Best practice:

• use 4 identical low-self-discharge NiMH AAs
• keep them as a matched set and charge/use them together
• ideally have a second matched set ready as backup

So: buy new rechargeables rather than relying on mixed old cells or disposable batteries.