Since the end of the Cold War, relations between the two countries have improved. Relations between the two countries have further improved since Laos joined ASEAN in 2000. However, Laos has always suspected Thailand of supporting anti-government Hmong activities (who were allegedly responsible for bombings in Vientiane) even though little evidence has ever been shown to support such an allegation. (During the Vietnam War the CIA armed ethnic Hmong fighters who fought alongside covert Thai and US forces against communist forces in Laos before the communist victory there in 1975.)
The Thai-Laotian Patrol Boat Incident (1980)
An alarming patrol boat shooting incident occurred in 1980...but this brief encounter was overshadowed by [later clashes in] Xaignabouri Province west of the Mekong. These conflicts originated in rival claims to forest resources based on maps from the early days of the French protectorate.
Source: http://www.onwar.com/aced/nation/tap/thailand/fthailaos1980. htm
Thai-Laotian Border Incidents (1984)
Following a series of shooting incidents in 1984 involving rival claims to three border villages... territory claimed by Laos as part of Botn District in Xaignabouri and by Thailand as part of Chat Trakan District in Phitsanulok Province.
Source: http://www.onwar.com/aced/chrono/c1900s/yr80/fthailaos1984.h tm
Thai-Laos Border War (1987-1988)
Fighting broke out in December of 1987 in a dispute over land claimed by Laos, which considered the territory as part of the Laotian Botn District in Xaignabouri and by Thailand as part of Chat Trakan District in Phitsanulok Province. Over 1,000 troops died before a cease-fire was put in place. Following the cease-fire in 1988, the two nations developed improved relations and increased trade.
Sources: http://www.historyguy.com/thai_laos_border_war_87.html