Biostar B450MHP v3 + Ryzen 9 3900 (non‑X): No CPU boost + random game crashes (stable in benchmarks)

I’m using this setup for almost 4 years:

CPU: Ryzen 9 3900 (non‑X)

Motherboard: Biostar B450MHP v3

GPU: Aisurix RX 5500 non‑XT 8GB

RAM: Lexar 2×8GB 3200MHz

PSU: Micronics Caslon BTC 700W 80+ Bronze (mining‑oriented unit)

OS: Windows 11 LTSC IoT Enterprise

Cooler: Deepcool AK620

Main issues:

CPU never seems to boost

The Ryzen 9 3900 never goes above its base clock (\~3.1 GHz). I’ve never seen it reach anywhere near the 4.3 GHz boost, even in games or heavier workloads.

Things I’ve tried:

Updated BIOS to the latest version available for this board

Loaded BIOS defaults / cleared CMOS

Enabled/disabled PBO and Core Performance Boost options

Checked Windows power plan (Balanced / High Performance)

Installed latest AMD chipset drivers

Monitoring with: \[HWiNFO64 / CPU‑Z / Task Manager – specify which\]

Random crashes in games, but not in benchmarks

While gaming, the system randomly freezes, black‑screens, or restarts if I don’t cap FPS.

Capping FPS to 60 mostly stops the crashes; anything higher and instability appears.

Benchmarks (e.g., Cinebench, 3DMark, etc.) usually run fine without crashing.

Goal

I’d like to fix this so I can either:

Use the PC reliably a bit longer, or

Sell it in working condition to help fund a second‑hand business laptop.

Right now it feels unsellable because of the instability.

My suspicion:

Given the board (entry‑level B450) and the PSU (Micronics Caslon BTC 700W, a budget mining‑oriented unit with a very loud fan and basic design), I’m wondering if:

The motherboard’s VRM can’t handle the 3900 well enough, causing no boost and instability under load.

The PSU’s aging 12V rail / basic design is causing crashes under fluctuating gaming loads but looks “okay” in steady benchmark loads.

Questions:

Is this a known limitation/bug with Biostar B450MHP v3 and Ryzen 9 3900 (non‑X)?

Could this combo realistically be held back by weak VRMs or a conservative BIOS that doesn’t allow proper boost?

What specific BIOS settings should I check/change for this board + CPU (PBO limits, CPU voltage, C‑states, etc.)?

Are there any concrete stability tests you recommend to confirm whether the CPU, motherboard, or PSU is the main culprit?

Any step‑by‑step troubleshooting advice or known fixes for this exact board + CPU + PSU combo would be a huge help. Thanks in advance. 🙏