Things to check:
- Nut: A slot that's too tight, rough or poorly cut can grab the string until it suddenly slips, often making it go sharp. Nut lubricant can help, but it's only a temporary fix for a badly cut slot.
- One string drifting: Inspect it for damage, check the installation and nut slot, and confirm it's the right length for your instrument.
- Bridge: Loose saddle screws, shifting saddles or strings not seated properly can cause instability.
- Setup: Neck relief, action and intonation all affect how the instrument holds pitch.
- Gauge or length: A big gauge change can shift neck relief, and the wrong winding length can keep a string from seating properly.
Weather matters too. Temperature and humidity make the wood expand and contract, so small tuning adjustments are normal, even after the instrument sits in its case. Retuning during a performance is normal as well.
Tune up to pitch. If you go sharp, drop below the note and come back up to take slack out of the system.