The FLASH effect has gained significant attention in radiobiology and radiation oncology due to its potential to improve therapeutic outcomes by delivering ultra-high dose-rate (UHDR) irradiations. Understanding UHDR biological mechanisms can also contribute to the development of biodosimetry and radiological medical countermeasures. However, achieving stable and reproducible high-current UHDR electron beams has been reported to be challenging with modified clinical linear accelerator (Linac) systems, and has not been systematically studied.We investigated how key standing-wave linear accelerator parameters, including electron gun current, pulse-forming network voltage, and auto-frequency control, affect the stability of electron beam intensity on a modified Varian Clinac 2100 C. We also developed a parameter-tuning method to adjust beam intensity and improve beam stability.This approach enabled (1) fine-tuning of dose-per-pulse without modifying the physical setup and (2) reduction of beam fluctuations, particularly during cold starts. These improvements enhanced both pulse-by-pulse stability and trial-by-trial reproducibility. The resulting stability was validated through multiple biological experiments.This work offers practical guidance for improving UHDR beam stability and reproducibility, as well as enabling intensity tuning in modified clinical linear accelerators. It can support the development of more reliable preclinical FLASH irradiators, thereby contributing to the advancement of FLASH research.