Background: Research supervision quality significantly impacts postgraduate student outcomes, yet supervisors' emotional and psychological characteristics remain under-examined, particularly in resource-constrained contexts. This study investigated fluctuating moods and ego-driven behaviors among research supervisors in Uganda's private universities and their effects on student experiences and outcomes. Methods: A mixed-methods convergent parallel design was employed across five private universities in Uganda during March-August 2024. Participants included 212 postgraduate students selected through stratified random sampling and 45 research supervisors purposively selected, along with 12 in-depth interviews with students experiencing severe supervisory challenges. Data were collected using validated questionnaires (Cronbach's alphas: 0.87 for students, 0.82 for supervisors) measuring frequency and intensity of mood fluctuations, ego-driven behaviors, and impacts. Quantitative data underwent univariate analysis (descriptive statistics), bivariate analysis (Pearson correlations, chi-square tests, ANOVA), and mixed-effects logistic regression modeling to account for hierarchical data structure (students nested within supervisors and universities). Qualitative data were analyzed thematically using NVivo software. Results: Descriptive analysis revealed high prevalence of problematic behaviors, with 52.4% of students frequently experiencing supervisors alternating between supportive and dismissive attitudes (M=3.42, SD=1.08) and 56.6% encountering methodological rigidity without justification (M=3.52, SD=1.14). Bivariate analyses showed supervisor workload positively correlated with both mood fluctuations (r=0.52, p