I have a Metz 58 AF-1 flash. I got it new in 2010 and it has worked great. Over the last 6 months, the recycling time has gotten consistently worse and worse. Before this issue, I could usually shoot for several hours and get over 100 flashes on one set of regular alkaline batteries with a recycle time of a few seconds (normal), but now I can put brand new batteries in and the recycle time is anywhere from 1-3 MINUTES (longer as time goes on)! Today, I put fresh batteries in and immediately got the "low battery" symbol on the screen and it wouldn't even flash at all. Has anyone ever seen this before? Thanks!
The very long recycle time indicates that at least one (if not all) of your batteries is bad/depleted. If you're using rechargeables, the chances are simply that they aren't holding charge well any more. That happens.
The easiest fix to try would be to get new batteries. If you like using rechargeables, I'd also recommend getting a "smart" recharger that lets you condition and test your batteries as well as charge them, so you'll know when to replace them.
If swapping batteries doesn't work, then it's possible there's an issue with the capacitor in the flash. I would not recommend attempting to fix this yourself, unless you have electronics skills and tools to discharge a capacitor (so you don't kill yourself; these things can still hold triple-digit voltages after the batteries have been taken out), and desolder/solder board components, and can source a suitable replacement part with enough capacitance.
Whether it's worth sending it to Metz for repair or purchasing a replacement is up to you.
Consider trying single-use, high-energy lithium batteries – as a troubleshooting measure to rule out inadequate energy as the cause of your problem. If the flash still does not function properly, it has likely reached end of life.
Possible causes of your problems:
Batteries have a limited shelf life, per Mark. They last longer if kept in cold storage, but must be brought to normal operating temperature before use.
Battery contacts could be corroded, per DrMoishe Pippik.
A component within the flash unit, such as a capacitor, may be damaged. Read somewhere that not using a flash unit for an extended period of time can cause problems.
Mismatched batteries (brand, age, charge-level) may not function well together.
Rechargeable batteries have a lower voltage and higher internal resistance, per jdv. Some devices have a setting to specify the battery type.
Newly purchased rechargeable batteries usually must be charged before first use.
Some rechargeable batteries may suffer "memory" effects (especially nickel-based batteries). Sometimes, memory effects can be reversed by fully discharging them in a low voltage device, such as a flashlight. However, some battery types are destroyed or damaged when fully discharged (lead-acid, lithium).
Many chargers recharge cells in pairs. If one cell has been drained much more than the other, the charger may be unable to fully charge it.
That sounds like something in the flash's charge/battery circuit is broken. You state "over the last 6 months, the recycling time has gotten consistently worse and worse" which indicates that it has been in regular use. Also you write "Today, I put fresh batteries in and immediately got the "low battery" symbol on the screen".
If the flash had capacitor failure, it would not show empty batteries, and the charging noise would frequently restart with a bang. Also capacitor failure is typical for flashes that go unused for long periods: that does not sound like it would be the case for yours.
Check out the battery contacts: are they clean and provide solid contact with the batteries? If some of the contact is through a screw or rivet, it is worth checking that it still is firmly in place.